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frex3wv

True North vs. Magnetic North

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Hey guys - I realize this may be a pretty simple question to answer - but its easier to post here then research out - so here goes...........

 

My Mag. GPS gives me the choice of True or Magnetic North.

 

I will never use this for geocaching (I don't even use the maps) but I DO want to use it for the sake of finding benchmarks utilizing the descriptions on the datasheets. So.... when the surveyor writes that a mark (either ref. or station) is this direction or that direction - which one of those two "north's" should I have my gps on?

 

See - told ya simple - but I do need to know....... and have been meaning to ask for months.

 

Thanks in advance. I hope you all are well!

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Hey guys - I realize this may be a pretty simple question to answer - but its easier to post here then research out - so here goes...........

 

My Mag. GPS gives me the choice of True or Magnetic North.

 

I will never use this for geocaching (I don't even use the maps) but I DO want to use it for the sake of finding benchmarks utilizing the descriptions on the datasheets. So.... when the surveyor writes that a mark (either ref. or station) is this direction or that direction - which one of those two "north's" should I have my gps on?

 

See - told ya simple - but I do need to know....... and have been meaning to ask for months.

 

Thanks in advance. I hope you all are well!

The azimuths given in the box score are always true relative to true north. The description part usually just gives compass points (NW, SE etc.) but occasionally will give deg/min/sec. Once in a while it will say magnetic. Assume true unless stated - but be prepared for exceptions. Sometimes they will give azimuths relative to south ("south 33 degrees west") so be careful of these.

 

I generally put the locations of reference marks into the GPS, so then it won't matter if you use true or magnetic (assuming you GPS will point a waypoint). If you use the GPS as a compass, and you use the box score, set it to read true.

 

NOTE: for scaled locations, none of this will matter, since the location may be off by a lot.

Edited by Papa-Bear-NYC

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In the verbal descriptions, I believe the directions are magnetic North. Surveyors for some reason only seem to write descriptions based on an 8-point compass. I prefer using 16-point compass notation (like NNE).

 

If you really want to be nerdy about it (even I would not do this...) you would need to take into account the polar drift (or whatever it's called) between the year the description was written and the present date, to see how different the declination is, by using the NOAA declination website. :o

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Use True for your GPS. When it has a place in a description the surveyor should tell you what version of north they are using (often Grid).

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ok - so is the concensus thus far "true" north on the GPS.

 

Here is the specific line in a description that finally made me ask the question:

 

"IN THE TOP OF THE SOUTH BACK WALL OF THE EAST ABUTMENT"

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frex3wv -

 

To be more explicit --

 

There are 2 sets of 'directions' on a datasheet:

1. The ones in the box score

2. The ones in the verbal descriptions (like your abutment example)

 

I figured you were asking only about #2 so I answered regarding #2 only.

I believe Papa-Bear-NYC answered regarding just #1.

(I have never seen an example of what Renegade Knight mentioned.)

 

So, to answer about both #1 and #2 --

 

The ones in the box score are certainly based on True North or True North datum.

The ones in the verbal descriptions are, in my opinion, based on Magnetic North.

 

Therefore, set your GPS unit accordingly depending on whether you're working on #1 or #2.

(Beware of the declination change over time when dealing with the Magnetic North data that's in #2.)

Edited by Black Dog Trackers

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There are 2 sets of 'directions' on a datasheet:

1. The ones in the box score

2. The ones in the verbal descriptions (like your abutment example)

 

The ones in the box score are certainly based on True North or True North datum.

The ones in the verbal descriptions are, in my opinion, based on Magnetic North.

Actually, I suspect that the ones in the verbal descriptions are based on the surveyor standing there saying, "Hmm, okay, the sun is over there and it's late afternoon, so south must be about, um, there..." :o

 

We won't even get into situations where the mark is between Object A and Object B and is described as being east of both of them... :o

 

Patty

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...So.... when the surveyor writes that a mark (either ref. or station) is this direction or that direction - which one of those two "north's" should I have my gps on?...

Surveyor's may have used a magnetic compass, but they generally refence True North. If magnetic is used, it would be stated as such.

 

...Surveyors for some reason only seem to write descriptions based on an 8-point compass...

Surveyors only use eight-point compass headings as a general direction for location. Surveyors use bearings broken into four, 90 degree quadrants - NE, SE, SW, & NW - based on True North. Surveyors also use azimuth bearings, which is essentially a one-point compass based on True North. On a day to day basis, most surveyors use a compass with 1,296,000 points. (DD MM SS = 360x60x60 = 1,296,000 points).

 

...you would need to take into account the polar drift (or whatever it's called) between the year the description was written and the present date, to see how different the declination is...

One reason (of many) why surveyors use True North.

 

...The ones in the verbal descriptions are, in my opinion, based on Magnetic North...

...Actually, I suspect that the ones in the verbal descriptions are based on the surveyor standing there saying, "Hmm, okay, the sun is over there and it's late afternoon, so south must be about, um, there..."...

In a situation like frex3wv described, that's probably what happened. It's relatively easy to determine which side of a road or canal is the easterly or westerly side (or northerly or souththerly side). It's highly unlikely that any surveyor got out a compass to determine the east abutment from the west abutment.

 

- Kewaneh

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Yes. Sometimes the verbal descriptions can be quite misleading, if not quite wrong! I'll go with Wintertime's hypothesis: ...Actually, I suspect that the ones in the verbal descriptions are based on the surveyor standing there saying, "Hmm, okay, the sun is over there and it's late afternoon, so south must be about, um, there..."...

A recent example is: KV6064. Everyone knows that the Delaware River runs North/South! Actually, it meanders back and forth. At Stockton, it runs WNW/ESE. The canal lock runs NW/SE. mr.magoo was a very good benchmarker. The description here got to him. The dirt road is at the ESE of the canal lock; not the south. Perception can cloud the mind. How often have I wondered where the south head wall of the east-west bridge was?

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Original Old surveys from what I understand were done by reference to Compass direction and the Magnetic Declination was ticked off on the compass dial.

They have one of the Magnetic Transits on display here in our Museum that they actually used in the 1800's.

 

I use True North on my GPS setting.

 

But if I am trying to work something out there is always the map datum as well.

If the map you are using has NAD27 on it then you need to use the NAD27 setting.

 

I just read an article on Joseph Brown and Prospeckt K. Robbins who were Early surveyors of the 5th Principal Meridian.

The article goes on to state there is a difference of about 200-300 feet between NAD27 and NAD83.

 

So if you are looking for something that was based on NAD27 and your GPS is set to NAD83 there may be some error.

 

I have also been looking into the Magnetic changes over the years and have found evidence that the Magnetic Field as well as the North Star are progressing West.

 

If you would like to go into the details of that though we will be here talking a long time.

And I am not a Profecsional Surveyor but I did stay at a Holiday Inn last night.

 

This is also just a Theory at the moment until I can prove it myself and then it will become a Natural Law.

Time will tell.

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Harry, your river directions are a lot like what I've seen along railroads. In many cases they wrote the descriptions based on the general direction of the railroad and ignored local twists.

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Harry, your river directions are a lot like what I've seen along railroads. In many cases they wrote the descriptions based on the general direction of the railroad and ignored local twists.

When I do work for my railroad, I will often use the terms "railroad north" and "railroad south". Obviously, "railroad north" is northbound on the line, regardless of which direction it may be at the specific location. (And with our line, that COULD be SW or SE!) That's bled over into a few recoveries I've submitted to the NGS as well.

 

An example of something would be, "The station is 100ft railroad north from milepost 423."

 

But I digress.

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I generally assume that if magnetic is not stated, true north is intended.

The declination in my area is 15° these days and I leave that correction dialed into my sighting compass. I expect no more than 2 or degree accuracy with that instrument.

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This station is one that bugged me quite a bit. The verbal directions were incorrect. The building has no northeast corner and even if it did, the other measurement would have to be through the building. I've just never found the verbal directions to be very accurate, and assumed the mark setters were using a hand compass or using an estimation like Wintertime and Harry Dolphin are saying.

 

I'm imagining that if surveyors were setting a bench mark they were not calculating true North at that location. (This would not be the case when setting a horizontal control mark, of course.) Is this incorrect?

I suppose they could approximate true North if they had a topograpic map handy and had its orientation matched pretty well with local surroundings. They'd be doing that anyway to make a scaled location, I assume.

(I'm not talking present day, of course - I'm theorizing about 1935 vertical control marks.)

 

It really does seem like reckoning without even using a simple compass is what was done. If a compass was used, 16-point directions would likely be used.

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Foxtrot,

 

I have yet to find a mark that referenced railroad direction instead of actual compass direction when describing a mark set beside the tracks. However, describing marks seems to be a personal thing and different surveyors may have done it different ways. I don't think I would find railroad direction to be very useful. I am big railfan and I would be hard pressed to tell you railroad direction on most of the lines in my area!

 

Another thing I have noticed about directions is that when they say "North of" some linear feature (road or tracks) they almost always mean perpendicular to the feature in a northerly direction as opposed to precisely north of the feature.

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Foxtrot,

 

I have yet to find a mark that referenced railroad direction instead of actual compass direction when describing a mark set beside the tracks.

Along the old NY central line through my area they almost all have a phrase like:

AT NEW YORK, IN BOROUGH OF BRONX, ABOUT 0.8 MILE SOUTH ALONG THE NEW YORK CENTRAL RAILROAD FROM THE STATION AT RIVERDALE, ABOUT 0.15 MILE NORTH OF MILE POST NY 12, ACROSS TRACK FROM POWER TRANSMISSION TOWER NO. 203, A MONEL METAL RIVET IN THE TOP AND 0.6 FOOT EAST OF WEST CORNER OF CONCRETE SPLICING CHAMBER NO. 926, ABOUT 40 YARDS SOUTHEAST AND ACROSS TRACK FROM POWER SUBSTATION NO. 12, 6.3 FEET EAST OF EAST RAIL OF EAST TRACK AND ABOUT 1 FOOT ABOVE LEVEL OF TRACK.
Note that the tracks are heading approximately NE at this particular point.

 

This mark was set by the RR and everyone working for the RR knows if it's a north/south line, an east/west line or whatever. Track directionality is second nature. Sort of like signs on an even numbered interstate (like I-84) which says "East" when you're heading north.

Edited by Papa-Bear-NYC

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This thread made me curious about how accurate decriptions like "IN THE TOP OF THE SOUTH BACK WALL OF THE EAST ABUTMENT" have been for the places I've visited. My gut feeling was that they are not very accurate, given the number of times I've spent puzzling about which end of the bridge and which wingwall a mark should be on. Here's a rogue's gallery of bridges that I've encountered:

 

KV3146.jpg

"IN THE TOP OF THE NORTHEAST END OF THE SOUTHEAST ABUTMENT" It would be clearer to call it the south end of the east abutment. It's hard to picture it as northeast of anything.

 

LY0814.jpg

"IN THE TOP OF THE NORTH WING WALL, 10.5 FEET SOUTHWEST OF THE SOUTHWEST RAIL OF THE TRACK, AND 9 FEET NORTHWEST OF THE CENTER LINE OF THE ROAD" It was clearly wrong to say it was "NORTHWEST OF THE CENTER LINE OF THE ROAD" and that mistake may have contributed to the "NOT FOUND" report in 1954.

 

LY0773.jpg

Originally "IN THE TOP OF THE SOUTHEAST END OF THE NORTHEAST ABUTMENT" which seems OK. Revised to "NORTH WEST END OF NORTH EAST ABUTMENT" by a later report. Uh, can we chip in and buy a fellow Geocacher a new compass? :o

 

LY0770.jpg

Originally "IN THE TOP OF THE NORTH END OF THE EAST ABUTMENT", which seemed fine. Revised to "IN THE EAST END OF THE NORTH ABUTMENT" by later report :D . I prefer the first, since there isn't really any north abutment on this bridge, only east and west ones, or perhaps northwest and southeast ones.

 

LY0745.jpg

"IN THE TOP OF THE NORTHEAST END OF THE SOUTHEAST ABUTMENT". That's more reasonable for the magnetic directions than for true north directions.

 

KV1261.jpg

"IN THE TOP OF THE SOUTHWEST WING WALL". Hardly. It's a real stretch of the imagination to call that southwest of anything.

 

LY0760.jpg

"IN THE TOP OF THE NORTHEAST END OF THE SOUTHEAST ABUTMENT" Again, magnetic north seems more useful here. Otherwise I would have called it the north end of the east abutment.

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I know what Holo means - I find myself reading over descriptions time and again - slowly..... trying to make heads or tails.

 

I did try for one yesterday with several other adults - and because I had the help - I didn't bother with the compass ( I am still reading over all the posts - and trying to decide what to do).

 

Here was the part of the description I was using:

 

IN THE TOP OF THE SOUTH BACK WALL OF THE EAST ABUTMENT, ABOUT 7 FEET SOUTH OF THE CENTERLINE OF THE TRACK, AND ABOUT 1.5 FEET LOWER THAN THE TOP OF THE LOW RAIL.

 

I went to the abutment I firmly believe they are refering to and used a metal rake to remove many years of debris and dirt - then used a broom. No benchmark - and I am baffled. Anyone have any idea what I may have done wrong and can anyone tell me what a "low rail" is? Should the term "back wall" be telling me something about it not being where I normally have found them?

 

I am stumped.

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I believe that all of these “points of the compass” directions are with respect to true north. In cases where magnetic north was used, it should have been specified. However, since only 8 points of the compass were used (nearest 45 degrees) in most cases the difference between true north and magnetic north doesn’t matter, see below.

 

For station description specifications, see USC&GS Special Pub. #247, “Manual of Geodetic Triangulation”

( http://docs.lib.noaa.gov/rescue/cgs_specpu...o247rev1959.pdf ), pages 118-120. Subparagraph (3) covers information for the “box” data and subparagraph (6) covers measurements to “definite objects in the vicinity such as witness posts, center lines of roads, fences, ditches, power poles, prominent trees, wells, houses, etc.” for the third paragraph of descriptions. Subparagraph (3), page 118, recommends true directions, (6) does not specify. In the text of descriptions, the points of the compass should fully be spelled out.

 

A map of magnetic declination is at: http://www.ngdc.noaa.gov/seg/geomag/img/us_dec_8x11.pdf . Note that for the lower 48 states, the worst case is about 15 degrees, although parts of Alaska are much higher.

 

GeorgeL

NGS

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holograph's experiences are pretty much the same as mine with AA2666 mentioned in my post above, and apparently similar experiences with several others of us.

 

(Note that I am speaking here in this whole post ONLY about the verbal descriptions, NOT what's in the 'box score'. We all know the 'box score' information is relative to (datum) True North.)

 

/rantmode on

 

I have pondered the last few years (since 2002) over repeated evidence that the directions given in the verbal station location descriptions I'm trying to read would've been vastly improved by the author's use of even the most basic drugstore or boy scout compass!

 

What is the deal I'm wondering? Part of the process of locating a bench mark (meaning the vertical control type of mark) is to give approximate coordinates. These are scaled, I have been assuming, from a topographic map that's in the hands of the people watching the cement being poured for the mark. Imagine yourself holding a topo map on the site - would you make those direction statements that holograph found in descriptions? I'm sure not! This makes me wonder about the typical level of accuracy of the scaled coordinates. If a mark is clearly South of something, but the description says Southwest, then what to think about the coordinates? Are these done by separate people; one in an office, perhaps?

 

I have been assuming all along - something along the lines that Wintertime described - and why would anyone do that level of reckoning with information painstakingly corrected to True North?

 

One could list 4 levels of determining True Norh directions (SW, S, E, etc.):

1. High tech (theodolite, GPS, etc) methods

2. Orienting a topo map with a compass (adjusted to local declination) and local landmarks

3. Orienting a topo map with local landmarks

4. Dead reckoning

 

Which was used, I wonder? It all too often seems like #4, sorry to say.

 

/rantmode off

 

Kewaneh, who is a surveyor, has said they're True-North based so that's what's being done now and in the recent past. What I'm wondering about, though, is what was being done in 1940, 1930, 1920, 1910....

I've been hoping DavdD or NGS Surveyor would come in here and let us know.

 

---------------

I had the above typed a while ago and then was distracted for a while and now I see that NGS Surveyor has come in with information. I decided to keep the rant, though.

 

From NGS Surveyor's analysis of paragraph 6, there is no rule on which to base a decision (like what compass mode to set your GPS) on whether something like "8.3 feet NW of a powerpole" is based on true north or magnetic north. Fortunately, for the 48 states, as NGS Surveyor pointed out, it just doesn't matter much whether one thinks of it as magnetic or true north. The unknown reason behind what I ranted about has more effect than that difference, as well.

 

I have yet to see an example of a description stating whether something like "8.3 feet NW of a powerpole" is based on true north or not. Has anyone? I haven't looked at Alaska PIDs.

Edited by Black Dog Trackers

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I think understanding the directions requires doing our best to put ourselves in the place of those who wrote them. The crew setting the elevation benchmarks along a rail line or road in 1934 was not highly equipped and instrumented, and their purpose in writing the directions was for the leveling team coming behind them in a few weeks to be able to find the marks they had set. It probably was not in the job description to write directions that would have everything we would need 70+ years later.

 

They may or may not have had a pocket compass, but in most cases probably didn't figure exact readings were needed. And in most parts of the country northeast is roughly northeast on either a true or magnetic scale. They read the truck odometer or counted telegraph poles or rail joints for distance, and it would make sense if they described the directions by mostly dead reckoning I think.

 

I have observed some tendency to use "railroad north" but never seen the term explicitly stated on the data sheets in my area.

 

There are other series of marks that were set under other conditions and one should not expect the USGS crew of the 1960's to be doing things the same way that a C&GS crew did in 1934, or for that matter the crews in 1934 in Michigan, Iowa, Arkansas, and South Caroline to be describing things in exactly the same way.

 

Part of the challenge it to get familiar with the style used in each series of marks, so that you can best deduce what that party of your grandfather's peers did and meant long ago.

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What is the deal I'm wondering? Part of the process of locating a bench mark (meaning the vertical control type of mark) is to give approximate coordinates. These are scaled, I have been assuming, from a topographic map that's in the hands of the people watching the cement being poured for the mark. Imagine yourself holding a topo map on the site - would you make those direction statements that holograph found in descriptions? I'm sure not! This makes me wonder about the typical level of accuracy of the scaled coordinates. If a mark is clearly South of something, but the description says Southwest, then what to think about the coordinates? Are these done by separate people; one in an office, perhaps?

 

Off the subject of True versus Magnetic North, but...

From what I've heard, transcription from topographic maps was done by office personnel, with transcription erros, and sometimed handwriting errors. I will usually check the topographic maps for an "X", and use the coords from Topozone, if there's a difference. I find it helps.

Albeit, not a major difference, here is the Topozone map for KV1250. The red X is the scaled coords.

 

ab1c23e9-e04d-4ca5-9b80-83fea6ad8378.jpg

 

N 40 52 05 W 074 52 58 Scaled Coords

N 40 52 05 W 074 53 00 Topo Coords

N 40 52 04.8 W 74 53 00.3 Hand-Held GPS Coords

I've seen far worse. I've never seen the scaled coords more accurate than the X on the map.

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Is it possible that in some cases we are dealing with a misunderstanding of the definition of parts of a bridge? To someone not familiar with bridges, the term "abutment" could be easily misunderstood. Even as an engineer (chemical & electrical), I don't think that I did before starting benchmark hunting. I do now. We have had this discussion here before, and there was a neat link to a site with pictures & words, but despite 10+ minutes of searching, I didn't have any luck. Here is one site I found.

 

So if someone didn't have a clear idea of what an abutment was, that could sure cause some difficulties. Of course, any surveyor would know. Good chance a casual geocacher finding an occasional benchmark might not. A 1934 crew setting discs for levelers to come along later? One would think so, but.....

 

Just a thought.

Edited by Klemmer & TeddyBearMama

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Are you looking for HX1952? If so, the Topozone map of the mark shows it as being on the southeast corner of the bridge (pretty much where I would expect it from the description. I am not sure about the description of the south wall or of the use of the term low rail. I would expect it to be pretty much where every abutment mounted disk was though, and I suspect that is where you already looked. I don't think it is vertically mounted as the description would most likely mention that, and a disk mounted vertically at that height would be unusable.

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For some of the ones that I pictured, where the road crosses the stream at an angle, it is natural to orient oneself by the direction of the road centerline, rather than by the relation that the abutments have to themselves and the stream. That explains some of the cases where an abutment may be called "southeast" when in fact the abutments are almost parallel north and south, but skewed.

 

Here are two bridge illustrations from the benchmark wiki. The nomenclature was confirmed by several engineering and state bridge standards documents.

 

bridge.png

 

suspension_bridge.png

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One term I would add to the excellent illustration at the top is "headwall" which is the outside edge of the deck--basically the edge of the bridge running from abutment to abutment. It is often mentioned on railroad benchmark descriptions.

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I have also run across the term "capstone." I believe that when I found a mark with this term it was in what I thought was the top of the abutment.

 

Thanks for the input mloser. I will have to return for another try at it.

Still wondering what the "low rail" reference is. Any RR buffs out there know?

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What is the deal I'm wondering? Part of the process of locating a bench mark (meaning the vertical control type of mark) is to give approximate coordinates. These are scaled, I have been assuming, from a topographic map that's in the hands of the people watching the cement being poured for the mark. Imagine yourself holding a topo map on the site - would you make those direction statements that holograph found in descriptions? I'm sure not! This makes me wonder about the typical level of accuracy of the scaled coordinates. If a mark is clearly South of something, but the description says Southwest, then what to think about the coordinates? Are these done by separate people; one in an office, perhaps?

Harry Dolphin gave some answers to this, but DaveD gave some more details in a post a while back.

 

Here's the thread: http://forums.Groundspeak.com/GC/index.php...=144529&hl=

 

Dave's post is the 5th one down.

 

The general steps were as follows:

 

1) A bench mark (that's a real bench mark I'm talking about) was set and and a verbal description was written down. No attempt to ascertain the exact location was made.

 

2) At some point (sometime in the mid 1970s.) the NGS decided to computerize the records and around the same time decided that having location and elevation data for all entries would be helpful to the public, especially with the rise in the use of GIS software.

 

3) The records were input (I think onto IBM punch cards) by clerks and the locations were supplied (by a separate "Cartographic" office) by others who had to read the descriptions and use rulers and topo maps to arrive at a location.

 

Similar steps were done for triangulation stations where the elvation was scaled.

 

Bottom line: the location (or elevation) data was added by clerks separated by decades from the original survey and by one or two levels of transcription from one medium to another.

 

DaveD: did I remember that about right?

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Are you looking for HX1952? If so, the Topozone map of the mark shows it as being on the southeast corner of the bridge (pretty much where I would expect it from the description. I am not sure about the description of the south wall or of the use of the term low rail. I would expect it to be pretty much where every abutment mounted disk was though, and I suspect that is where you already looked. I don't think it is vertically mounted as the description would most likely mention that, and a disk mounted vertically at that height would be unusable.

This brings up an important point: whenever I have found a mark located by the USGS on a map (usually by a small "x") the position shown is much closer to the real position than one I could deduce from the description and also much better than the scaled coordinated from the datasheet. I can only assume the the USGS actually recovered these marks and used them to control their maps.

 

I think that many of us (certainly myself) forget to look at a topo map and see if a bench mark we're looking for may actually be shown there. Note to self: "look at the map before you go".

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I think headwall and capstone are usually features of arch bridges and tunnels. Here are the terms as I've seen them used.

 

bridge3.jpg

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Still wondering what the "low rail" reference is. Any RR buffs out there know?

 

Is there any chance that the bridge is actually a viaduct carrying one railroad over another (or a former railroad)? In that case, one could interpret low rail as the set of tracks underneath the viaduct. Or perhaps there were at one time a set of double tracks not at the same level, one upslope and one downslope.

Edited by holograph

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Papa-Bear's advice is sound. I would love to say I always look at topo maps before I go hunting but that would be a lie. However, when I return from looking for a mark that I haven't found, but by all indications should have been there (for instance, the bridge doesn't look like it changed but no mark was evident), I will often take a look at the topo to see if I was in the right place, or even on the right bridge! I too have found that the X indicating the bench mark is usually right on, at least within the constraints of map scale and printing.

 

Holograph, you are right about headwalls. They are usually part of arched bridges, and when they are mentioned in bench mark descriptions there is either no capstone or the capstone seems to be considered part of the headwall. I think I have seen headwalls on non-arched bridges though.

 

Frex3wv, I have been a railfan all my life and can't recall hearing the term "low rail". If it was on a curve and the tracks were banked then one rail would be lower than the other, but that isn't very likely in this situation, first because the bridge isn't on a curve and second because banked curves are usually only seen on higher speed rail lines (there are some great banked curves on Amtrak at Gap, PA, east of Lancaster, where the difference between the inside and outside rails is quite a few inches). I wouldn't put too much stock in the term though. It could be a mistype or some odd interpretation of what the surveyor saw.

 

Holograph, the topo map just shows the railroad going over a creek.

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I would also like to confirm Papa-Bear's advice about looking at the topo map. Over on the Waymarking side, there have been some USGS disks waymarked in the sub-category of find called "searched @ BM on a Topo map" (for non-NGS marks). Just to see the concordance, I typically click on the topo map and the GPS-measured location is often right on the mark.

 

More to the point, though, if it is not right on the mark, there is likely an effect that happens in which the BM mark is where it looks like it should be, relative to the nearby landmarks on the topo map, even though it is not at the correct coordinates. Another way of saying this is that if a topo map is showing a bridge (that the mark is on) at coordinates X with respect to the tickmarks along the edge of the map, but that bridge is actually at coordinates Y, then the coordinates on the datasheet would be X, but the actual coordinates of the mark is Y. This concept was pointed out a while ago by Wintertime. It is very similar to the idea that, for true bench marks, one should pay more attention to the landmark orientation than to the listed coordinates, except with a topo map, the landmark orientation information is in the form of a drawing, not "the mark is 83 feet SW of item Z".

 

Therefore, looking at the topo map likely adds siginificantly to the location information, assuming the mark is indicated on it.

 

One thing that mixes this whole concept up a bit, unfortunately, is that the location my have been done on a 15-minute map, not a 7.5 minute map, in which case getting the 15-minute map is better.

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I don't know if this helps at all or not but this discusses high and low rails a lot.

 

If a curve is banked then there is of course a low and high rail. I don't know about a banking on a bridge, though - that seems odd.

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Scaled Coordinates: who, how, when?

... What is the deal I'm wondering? Part of the process of locating a bench mark (meaning the vertical control type of mark) is to give approximate coordinates. These are scaled, I have been assuming, from a topographic map ...
... DaveD gave some more details in a post a while back. Here's the thread: http://forums.Groundspeak.com/GC/index.php...=144529&hl=

I figure I should do what I should have done in my post above, namely put DaveD's remarks in. Here they are:

 

NGS began constructing the foundation of the current database in the mid-1970's. At that time the data for stations was published as separate horizontal and vertical control documents. As part of the data frame it was decided that all monumented control points would have a position and an elevation, even if one of those values needed to be scaled from a map. At that time NGS had a small cartographic section that was tasked with plotting every station in the network (more than 900,000) on USGS 7.5 minute, 1:24,000 topographic sheets when necessary to get the elevation or position. In some cases this was easy because USGS already showed the mark on the topo map. In many others, the cartographer was required to read the description and follow the directions along the map to plot it as accurately as possible. Because the subjectivity of this process NGS decided to label the quality of scaled horizontal values to +/- 6 arc seconds. Since points with a known latitude and longitude were easier to plot we were generally confident to be able to scale the elevation for points that required that component to within 1/2 a contour interval. Having at least a scaled value for bench marks provides spatial information for computer mapping software that allows us to accurately assess the density of control stations in a given area and provides approximate information for surveyors to locate these marks. Scaled elevations on horizontal stations provides a check on the reduction of certain types of survey measurements that are fundamental to land surveying operations.

Thanks again Dave for always having the answers.

Edited by Papa-Bear-NYC

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ok - so is the concensus thus far "true" north on the GPS.

 

Here is the specific line in a description that finally made me ask the question:

 

"IN THE TOP OF THE SOUTH BACK WALL OF THE EAST ABUTMENT"

 

The following description may help with the terms "Back Wall" & "Low Rail".

 

Quote "Differential track support between left and right rails: One of the major factors contributing to the bridge transition issues at the eastern mega site is the skewed bridge back wall (abutment). These skewed back walls are there because the bridges cross rivers or roads not perpendicular to the railroad tracks. In general, the skewed back wall requires that approximately five ties at the transition area span both the bridge structure and the approach track. This creates a differential in support between left and right rails, i.e., the side supported on the abutment is a lot stiffer than the side supported on the ballast. Moreover, ties on the weaker side are often supported directly on the ground without ballast . As a result, shimming is often required to compensate for the track settlement on the weaker side. Another problem is the difficulty of conducting ballast tamping, because on one side ties are supported on the abutment and on the other side ballast particles often roll out."

 

Hope this helps clear things up a bit.

 

John

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