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U S G S Map Control


Papa-Bear-NYC

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I was wondering about control points for the typical USGS maps.

 

Is each little triangle and little x an actual control point used to produce the map, or are they just printed like everything else to show someone the (approximate) location of a feature?

 

The little "x BM" with an elevation seem to be much more accurately placed than the location given on NGS datasheets for bench marks (vertical control points). Did someone actually do a field check of these things when the map was surveyed?

 

An interesting thing I noticed in looking at one of the old 15' series maps of northern Maine was a statement at the bottom "Control by USGS, USC&GS and International Boundary Commision, Topography in the United States from aerial photographs by photogrammetric methods". I supposse there is something like this on all maps.

 

What does this mean? Is the control both vertical and horizontal, and the photos fill in the "shape" of the land (i.e. the contour lines)? It would seem to imply that they would need to know exactly where a bench mark is horizonrtally, not just it's elevation.

 

The Topozone guy was on this forum once, I bet he would know this stuff.

 

Thanks

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Have you seen this USGS Page?

Z15

 

Thanks for the link. Yes I have seen that before, I forget where.

 

It notates the vertical and horizontal control points, but it doesn't really answer my question whether the bench marks are field checked as to their location. I guess if they are control points they control the map (a truism) but it seems like you need to know exactly where a BM monument is (as well as the elevation it represents) to do its job of control.

 

This is a different function from the NGS datasheets for these monuments which is to provide surveyors with vertical control, but for which it is relatively unimportant to specify exact location (Lat/Lon). The surveyor knows he needs to level from the BM to his project; he doesn't particularly care what the USGS map of the area shows. He does of course care what the state, city, or county property maps show.

 

Harry

 

I agree the little X marks are almost always a better indication of the location than the datasheet coordinates. But I'm not sure the NGS scaled from the Xs on the maps (which after all exist for only a fraction of NGS BMs) or rather just scaled from their interpretation of the description provided when the thing was monumented. Some are so far off I would guess they ignored the X on the map. Or perhaps they scaled the coordinated before the X got onto the map. It's what the USGS (as opposed to the NGS) did that is more pertinent to my question.

 

This question arises both from curiosity and also from a mark I can't find. The scaled location given on the datasheet, when displayed using Topozone, is just where the description would put the mark, but I can find no mark there. OTOH, the USGS has a mark a block away on the map (which I haven't checked yet). So I'm thinking the datasheet may have the wrong street name due to human error or a mix up. If I can get out today, I'll let you all know. Of course this won't really answer the question I posed.

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... it seems like you need to know exactly where a BM monument is (as well as the elevation it represents) to do its job of control.

 

You need to know its position within the accuracy standards of the map you are producing. The USGS 1:24,000 topo maps have accuracy standards of about 45 feet, if I recall, meaning that 95% of the features plotted on the map will be within 45 feet of their true position. I'm sure the USGS field surveying crews aspire to more accuracy than that, but by the time it gets published on the map, the error is allowed to be that much.

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I use the seamless National Geographic interactive TOPO USGS map series.

I have found all the little X's,BM and features are much more accurate than any other program,map or deduction.

I use and have used it for recovery of the Control points,chisled squares,and benchmarks in several quadrangles now.

 

If you remember I wrote and asked for the field notes for my quadrangles,which are USGS data sheets.

 

These can be obtained through USGS Rolla, Mo.

Or they use to be.

 

I use the coordinates from the center of the cursor.

And create a waypoint.

I then redefine them in GPSTrackmaker to the 11 decimal points and then view in Google Earth Plus..

It also gives you elevations.

Which in a 3d world is a must.

 

You can believe me or not on this one.

But if you are at an elevation of 4000 feet and your GPS has an elevation of 0 an error is inherent.

 

Any how I do this all the time.

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It notates the vertical and horizontal control points, but it doesn't really answer my question whether the bench marks are field checked as to their location.

 

Yes most were field checked. They had plotters out in the mobile field offices. We had a person on our DOT crew who had worked mapping with USGS back in the 1960-70's. They had draftsmen who took the field data and drew up the basic maps (not like the finished product) then when aerial mapping became the standard they used aerial photos to field check stuff. Most of the marks, section corners and the like were searched for and plotted on the field copy as they found it. Have you every seen what is called a "Provisional Map"? On it you will find all the field notes by the crews. I have some here and when I have time I will scan a typical portion and post it

 

I agree the little X marks are almost always a better indication of the location than the datasheet coordinates. But I'm not sure the NGS scaled from the Xs on the maps (which after all exist for only a fraction of NGS BMs) or rather just scaled from their interpretation of the description provided when the thing was monumented. Some are so far off I would guess they ignored the X on the map. Or perhaps they scaled the coordinated before the X got onto the map. It's what the USGS (as opposed to the NGS) did that is more pertinent to my question.

 

NGS used/s the USGS maps to plot their BM's while they are found or being set but they they do this on every mark, who can say unless you were there when it was done. They may even use Delorme software as the state advisor had that on this notebook and often sent us those printouts when he wanted us to go reset a mark or do some recon to save him a trip. Back in 1996, I ran into a NGS BM recon/install team who were working on leveling to CORS stations and what he called "Harbors of Refuge" control. He showed me what was a B&W photocopy of a USGS map onto which he had all the BM's they recovered notted and the new ones they had set plotted. This was needed for the desriptions and as well as for the level crew which was working behind him.

Edited by Z15
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I just did a quick and dirty comparison of 10 marks that had scaled positions, and compared my GPS reading at the station with the datasheet's scaled location, and then with the location found from the USGS topo quad. The stations were LY0381,LY0551,LY0863,LY0864,LY0553,LY0545,LY0554,LY0505,LY2278, and LY0502.

 

Here are the datasheet errors for the stations (in feet from the GPS reading)

1115, 99, 151, 98, 73, 57, 13, 215, 100, 49

 

Here were the corresponding map errors (feet from the GPS reading)

118, 59, 127, 129, 173, 157, 171, 102, 71, 147

 

Half of the marks were closer to the datasheet's scaled location than to the map's plotted position, and the other half were closer to the map's position. The interesting thing is that the maps were more consistent, whereas the datasheet had some wild variations. The worst datasheet error was 1115 feet, but the worst map error was 173 feet. The best datasheet error was 13 feet, while the best map error was 59 feet.

 

Those stations were all clustered in a small region, and most were in one quad. It isn't really a scientific sample or analysis, but it tends to reinforce our perceptions that the maps can be more reliable than the datasheets for scaled positions.

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Q: An interesting thing I noticed in looking at one of the old 15' series maps of northern Maine was a statement at the bottom "Control by USGS, USC&GS and International Boundary Commision, Topography in the United States from aerial photographs by photogrammetric methods". I supposse there is something like this on all maps.

 

A: In order to build a series of maps that will fit together from boarder to boader, USGS required geodetic control points spaced at fairly regular intervals. Since the topography and planimetric features of the maps would be determined using aerial photography you need to have "control" points on the ground that will allow you to properly define the curved surface of the Earth onto a flat map. Depending on the requirement for positional accuracy, the requirements for horizontal and vertical control can be different. In order to reduce the costs of surveys, USGS would use control points from numerous agencies that could provide control points of sufficient accuracy, such as USC&GS/NGS, Corps of Engineers, International Boundary Commission and a few others.

 

Control points set for vertical control, often referenced as Bench Marks are established by the technique of leveling, while horizontal control points were determined by the methods of triangulationa and traverse, now replaced by GPS. The results have been that many hundreds of thousands of marks have highly accurate vertical but not horizontal or highly accuate horizontal but not vertical.

 

In the late 1970's as NGS was building the integrated database we made a decision that we would publish some value for each component for all marks. If a mark had been surveyed both horizontal and vertical then it's position is Adjusted into the respective datums. Stations with well determined horizontal would have their elevations scaled from the USGS topo maps and marks with well determined vertical would have their horizontal coordinates scaled from the USGS topos. The typical accuracy for a scaled height should be not worse than 1/2 the contour interval of the maps while the accuacy of the horizontal position is more complex. If the mark is plotted on the USGS topo then it's position could be scaled within about 1 second, if as occured in most cases the cartographer had to read the description of the mark and make a best estimate of it's horizontal location then the coordinates can be much more in error. This is why NGS says the expected error of the horizontal scaled position is approximately 6 seconds.

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I dare say over the years the maps were compiled by USGS there were thousands of people working on them and methods used varied from year to year and project to project and crew chief to crew chief.

 

Once upon a time we were working (circa 1982) in an area where there was a big USGS field crew mapping for 7.5 min maps in the Upper Penisula of Michigan. Was a big crew as I recall many green Dudge pickup trucks roaming the back roads of upper Michigan for the better part of 2 yrs. What I observered were mostly 2 man crews, a instrument man and a rodman. The rodman had a small off road motorbike and the instrumentman a a 4x4 pickup truck. They were running elevations up and down every road and trail they found. Once they were working in an our project area and we provided them with check elevations along a stretch of state hiway in which they had little control. We also filled in some missing section corners and C&GS BM's. I recall all he did was take out his mosaic and plot the NGS mark and section corner on the map by observing the contours and maps features, like "Well it looks to be right about here and placed a mark on his field map, sort of like what those who were doing mapping updates on a volunteer basis for USGS Map Corps.

 

They also had a leveling crew setting some new marks in areas where there was little control and of what I recall of the crew as that they were mostly seasonal workers. As Mike S. (an out of work at the time Forester), a former USGS employee (who I worked with me from 86->02) often told us about USGS Bench Marks was they were set for thier use and they did not care what anyone else was going to use them for. As long as they served thier purpose, it was good enough.

Edited by Z15
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Here is a typical example of what the field crews did in the circa 1982 field checking here, you can see the BM notatations. This is USGS as I referenced in my last post..

 

Br. - Bridge

W.L. - water level

BM'sd are noted as such with X

X - show spot elevations taken

 

Note--I noticed inconsistances on other maps as to how BM's are noted and likely due to different person doing the work. Some show the elev as this one and other give the designations, e.g A 123. Note-Only BM's found are noted...

 

Hopes this provides some info on the subject...

 

example1.jpg

Edited by Z15
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We'll go back to my old bugaboo: KV1232 Rockaway

Hopefully, this will work. Topozone map

The actual location of the benchmark is in the upper left BM 534. Topozone has it marked accurately. NGS pages have it about 1.9 miles off. 1 minute of latitude and 1 minute of longitude. An interesting error. I hae not actually found the benchmark. Additions were added to the Rockaway Borough Municipal Building. If the marks still exists, it is inside the building. I do need to call the municipal clerk some day, and see if I can get an appointment to look for it. It was on the back door sill.

But the point is that NGS is almost two miles off, but the topographic maps have it located accurately. And that does not answer Papa Bear's question. Obviously a transcription error.

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You're not crazy, Harry. KV1232 has travelled around the countryside in the recent past.

 

GC.com has it listed as 40° 52.983 W 074° 29.750, or N 40°52'59" W 074° 29' 45", which was presumably its position in 2000.

 

The Feb 2005 annual archive at NGS had it listed at N 40° 52' 59" W 074° 29' 46", when it shifted 1 second to the west, not much, but not really explicable by GC.com rounding errors, either.

 

The current datasheet has it listed at N 40° 53' 59" W 074° 30' 41", where it crawled 1 minute north and 55 seconds west.

 

Pretty soon this adventurous little mark will be in New York! I suggest that the next time you go looking for it, you "lead" a little in your sights and try to catch it on the run. :rolleyes:

 

FYI, here's a visual comparison of the scaled coordinate, the best guess actual coordinate, and the USGS Topo coordinate, overlayed on the NJ 2002 1 foot resolution DOQ of the area. The small red x marks the best guess location at the courthouse before the addition was built, and the other red x in the river marks the datasheet scaled coordinate. The light yellow lines are the location of the road, courthouse symbol, and the benchmark X on the USGS topo.

 

KV1232_doq.jpg

Edited by holograph
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Hmmm.... a questions creeps into my mind:

When NGS adjusts marks from the original Datum used when a mark is set, possibly NAD27 (?) in this case, to the current standard NAD83, are scaled marks adjusted as well? One would think so..... So if the mark was reported originally in the wrong datum, then adjusted..... Ahhhhh... then the error was caught and fixed.... or something like that.

 

It seems that the magnitude of the error (at least in my SoCal area) might be consistent with an NAD27 to NAD83 error. Just a thought.

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The most likely answer is that someone noticed the error in the scaled position some time ago and reported that to the NGS database team who corrected the values. All marks in the NGS database with a "Scaled" value in either horizontal position or elevation were determined by a cartographer manually plotting the location of the marks and transfering that information to a form from which the DB was created. Unlike the geodetic measurements (ADJUSTED) which contain redundant measurments to try and reduce errors, hand copied transcrption errors were unfortunately, not uncommon.

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