A few benchmark questions. Food for FAQ.

[Parsa]

Hi, I'm a fairly experienced geocacher, but just started benchmarking. I've been a map geek for much, much longer, so I knew about control points and markers. I've also tutored kids for topo map activities in Academic Decathalon.

 

However, I have some questions now that I've started that some of you surveyor types might be able to answer. Forgive me in advance if any of these have been asked before.

 

1. Some vertical and horizontal control points are marked (and even named) on my local topos that don't appear on the full "alive and dead" Geocaching/NGS list of benchmarks. The topos aren't incredibly new or old (c. 1970s revision, etc.), so why would that be?

 

2. What's the difference between a vertical control mark with "recoverable mark" and a spot elevation on the topo map? Both have the elevation and the X on the map. Is one black and one brown? What about the black elevations with no X at all (like at intersections)?

 

3. Why on earth would a contractor or other builder destroy an official federal benchmark for some trivial construction? I'm not talking about a major road widening, I mean like putting in a sidewalk or a horseshoe pit.

 

4. I went to a major horizontal control mark in my town (the designation is the town's name, Escondido), and it had the main station disk, reference mark Escondido No 1, reference mark Escondido No 2, a post, an azimuth mark, and somewhere there was an underground marker. If I only found the brass circle of Reference No 1, is it a find? It was at least right on the 0.0 point (NAD1983), and seems to exactly coincide with the triangle on the topo.

 

5. That leads to a related question. Why are the NAD1983 coordinates often *way* off? Is it only true on the vertical marks? How did they determine coordinate locations for the control points? Were some control points surveyed and located with more accuracy using astronomical and chronological measurements (such as on meridians)?

 

6. Do towers, and structures that act as control points have a disc somewhere, or is it just the structure that acts as the marker? If that structure is obviously nowhere near the coordinates (ex, a tall radio tower), is it "destroyed"?

 

7. If a marker is lost, and then in a later year listed as "recovered" and "good", why does it appear in the "dead" category (with a skull)?

 

 

Thanks a lot,

Parsa

[Z15]

You asked a lot. May take several years to answer all these questions fully but I will touch of some I know about.

 

1. Some vertical and horizontal control points are marked (and even named) on my local topos that don't appear on the full "alive and dead" Geocaching/NGS list of benchmarks. The topos aren't incredibly new or old (c. 1970s revision, etc.), so why would that be?

Well, if often boils down to when the maps were made. When some were made either the survey infomation was not avaialble to them or someone in charge made a decision that it was not needed on the map. Most of the control information was really not even published until the 70's. Prior to that it was in field books etc and retained by the agency doing the Surveys. The current maps were field checked in some instances and the party doing the work located the marks and marked them on the maps. Also, ones that are not marked could have not been in place at the time of the map. I am sure there is more to this but thats the jist. A lot of the USGS maps for my area were mapped in 1946 and photo revised in the 1970's.

 

2. What's the difference between a vertical control mark with "recoverable mark" and a spot elevation on the topo map? Both have the elevation and the X on the map. Is one black and one brown? What about the black elevations with no X at all (like at intersections)?

A spot elevation is just that, the level party determined an elevation of that point to the closest 0.10 meters to aide in mapping contours and for the map user. The vertical control mark is exact and it monumented, the spot elevation is not monumented. There is a pamphlet available from USGS that describes all the map symbols etc.

 

3. Why on earth would a contractor or other builder destroy an official federal benchmark for some trivial construction? I'm not talking about a major road widening, I mean like putting in a sidewalk or a horseshoe pit.

First of all, its the equipment operator that in all likelyhood was the culprit. They often do not even know they existed and I am afraid to say some just don't care. There are A.H.'s in every profession and would run over your car if it was parked in thier way. I know of instance where poeple found these on property the owned, were afraid of the gov't claiming their land or some other hair brained idea and they destroyed them out of contempt.

 

4. I went to a major horizontal control mark in my town (the designation is the town's name, Escondido), and it had the main station disk, reference mark Escondido No 1, reference mark Escondido No 2, a post, an azimuth mark, and somewhere there was an underground marker. If I only found the brass circle of Reference No 1, is it a find? It was at least right on the 0.0 point (NAD1983), and seems to exactly coincide with the triangle on the topo.

No, I would say not, you only found a reference, No. 1 and No. 2 are reference marks. The main purpose is to locate the control station in the event the surface mark cannot be found. The underground mark is at the same location as the station mark, just buried some ? feet down as a safe gaurd from loosing the station position. The azimuth mark is set for local surveys so as to have a bearing between the station and the azi to base a survey on. Surveying is the science of measuring the land (earth) and that is done by angles, distance and elevation. Most all surveys and property deeds use bearing and distance to locate and describe them. Often called "meets and bounds". Pick up any survey and you will find a bearing on it, S 80 deg 45 min 23 secs W 1240.02 feet, etc. The surveyor can use these 2 points to get on the state plant coordinate system for his state.

 

5. That leads to a related question. Why are the NAD1983 coordinates often *way* off? Is it only true on the vertical marks? How did they determine coordinate locations for the control points?

You have to read the data sheet, it explains a lot. First of all, GPS is only accurate to + or - 100 meters 90% of the time. Hand units while appearing to be right on are not at all accurate for surveying in sense othe than recon. Look at the data sheet and if it says "Scaled" after the lat and lon then it means, the survey crew plotted it on a USGS map and interpolated the position from that. There was no need to conduct a more precise survey for this point, it has only vertical importance but the scaled Lat and Lon could have some value in planning future work or finding your position on the ground. Without a scaled position, it could be anywhere but this pin points its location.

 

Were some control points surveyed and located with more accuracy using astronomical and chronological measurements (such as on meridians)?

Yes they were precisely surveyed by various methods, from triangulations to travese to GPS. The data sheet tells you how it was done. Classical Geodetic Methods are triangulation and traverse, while GPS is used exclusively today since the 1980's.

 

6. Do towers, and structures that act as control points have a disc somewhere, or is it just the structure that acts as the marker? If that structure is obviously nowhere near the coordinates (ex, a tall radio tower), is it "destroyed"?

No the towers etc are just landmarks, no disks exist on/in the landmark itself. But if it is a tower such as a old fire lookout lookout tower, it may well have an disk set in the ground under the tower, as often done by USGS to map the exact locations. You can't assume because your coords do not jive that anything is destroyed. Many things come into play here, maybe you have the wrong tower, or a new one was contructed close to the old position, your GPS receiver is not adequate, you have the wrong coord system or something else.

 

7. If a marker is lost, and then in a later year listed as "recovered" and "good", why does it appear in the "dead" category (with a skull)?

 

Don't know the reason for that but my just be a computer programming error. Also since someone reported it as lost and know it is reported as found, adding doubt.

 

This bears saying, HAND HELD GPS UNITS ARE NOT SURVEY GRADE AND WILL NOT GIVE THE SAME RESULTS. People assume the GPS is right on but that is not true. Its close! but not perfect. Thats why you need a professional surveyor to determine a legal position. If your are off 1 sec in position, that can equte to approx 100 feet on the ground. I have had some real battles with some persons who believe what they see on the hand help GPS reciever is right on the money. Most units are set to work in Degrees and decimals there of. A surveyor would work in degrees minutes seconds, e.g. ddd mm ss.sssssssss

 

to be continued I'm sure......

 

[This message was edited by elcamino on January 17, 2003 at 10:16 AM.]

[Parsa]

quote:

---

2. What's the difference between a vertical control mark with "recoverable mark" and a spot elevation on the topo map? Both have the elevation and the X on the map. Is one black and one brown? What about the black elevations with no X at all (like at intersections)?

 

_A spot elevation is just that, the level party determined an elevation of that point to the closest 0.010 meters to aide in mapping contours etc. The vertical control mark is exact and it monumented, the spot elevation is not monumented. There is a pamphlet available from USGS that describes all the maps symbols etc._

 

Yes, I know the technical difference, but one symbol sheet I looked at online had the same type of symbol for both. I'll have to dig up my detailed symbol sheets.

 

4. I went to a major horizontal control mark in my town (the designation is the town's name, Escondido), and it had the main station disk, reference mark Escondido No 1, reference mark Escondido No 2, a post, an azimuth mark, and somewhere there was an underground marker. If I only found the brass circle of Reference No 1, is it a find? It was at least right on the 0.0 point (NAD1983), and seems to exactly coincide with the triangle on the topo.

 

_No, I would say not, you only found a reference. No. 1 and No. 2 are reference marks. The main purpose is located the control station in the even the surface mark cannot be found. The underground mark is at the same location as the station mark, just buried some " feet down as safe gaurd from loosing the station. _

 

Thanks. I'll look again to see if I can find the main station. I was just happy that I was exactly at 0.0 on the brass reference disk. It looks identical to any other federal disk.

 

5. That leads to a related question. Why are the NAD1983 coordinates often *way* off? Is it only true on the vertical marks? How did they determine coordinate locations for the control points?

 

_You have to read the data sheet, it explains a lot. First of all, GPS is only accurate to + or - 100 meters 90% of the time. Hand units while appearing to be right on are not at all accurate for surveying in sense othe than recon. Look at the data sheet and if it says "Scaled" after the lat and lon then it means, the survey crew plotted it on a USGS map and interpolated the position from that. There was no need to conduct a more precise survey for this point, it has only vertical importance but the scaled Lat and Lon could have some value in planning future work or finding your position on the ground. Without a scaled position, it could be anywhere but this pin points its location._

 

Thanks for this explanation. I wasn't refering to GPS error. An error of 20 feet wouldn't concern me. I'm taking about 1 or 2 city blocks! You're explanation of "scaling" probably explains the gross errors.

 

6. Do towers, and structures that act as control points have a disc somewhere, or is it just the structure that acts as the marker? If that structure is obviously nowhere near the coordinates (ex, a tall radio tower), is it "destroyed"?

 

_No the towers etc are just landmarks, no disks exist on/in the landmark itself. But if it is a tower such as a old fire lookout lookout tower, it may well have an disk set in the ground under the tower, as often done by USGS to map the exact locations. You can't assume because your coords do not jive that anything is destroyed. Many things come into play here, maybe you have the wrong tower, or a new one was contructed close to the old position, your GPS receiver is not adequate, you have the wrong coord system or something else._

 

The benchmark in question is not shown on my topographic map. It was the radio mast for a radio station (KOWN) in Escondido, CA that no longer exists. The tower was a 150 ft red and white tower with a red light. There is no such tower within miles. Perhaps a disk exists in the ground somewhere, but the coordinates put me in the middle of an industrial building. Even the address on Hale Ave no longer corresponds to the numbering system. To anyone looking for the benchmark in the 60's or 70's, the mast would be obvious, but there's no detailed description to find where it once was.

 

---

[Z15]

Parsa

 

quote:

---

The benchmark in question is not shown on my topographic map. It was the radio mast for a radio station (KOWN) in Escondido, CA that no longer exists. The tower was a 150 ft red and white tower with a red light. There is no such tower within miles. Perhaps a disk exists in the ground somewhere, but the coordinates put me in the middle of an industrial building. Even the address on Hale Ave no longer corresponds to the numbering system. To anyone looking for the benchmark in the 60's or 70's, the mast would be obvious, but there's no detailed description to find where it once was.

---

 

If this is the case and you are without a doubt positive its gone then report it as not found. There is not disk. Often times the surveyors never even visited the location of most these landmarks like radio towers or church spires. Someone from the recon team may ahve gather the info but thats about it. They observed these from thier control stations many miles away for the most part and computed the position by triangulation. That is intersecting them from other point and by solving the triangles, determine position of the landmark.

[+embra]

elcamino, you are the energizer bunny of benchmarking...thanks for all your comments!

 

I would add to the question about whether the reference mark disks are finds or not that sometimes they show up in the database with their own PID. If so, you've got a find here. If the RMs are only mentioned in the descriptions for the station marks, what elcamino said covers it.

 

Don't get even - get odd

[+rogbarn]

quote:

---

Originally posted by Parsa:

1. Some vertical and horizontal control points are marked (and even named) on my local topos that don't appear on the full "alive and dead" Geocaching/NGS list of benchmarks. The topos aren't incredibly new or old (c. 1970s revision, etc.), so why would that be?

---

 

Keep in mind that there are two completely different agencies that we are talking about here. The USGS makes topo maps and does the field work necessary to do that. They do place monuments which sometimes, but certainly not always, get added to the NGS database. The NGS (part of NOAA) maintains the geodetic survey of the US, places their own markers and maintains the database of control points. Their database includes monuments from many federal, state and private agencies, companies, etc and must meet their strict guidelines. Actually, I'm not sure if the NGS is still out placing many markers anymore, but they certainly have placed many, many markers in the past.

[+rogbarn]

This thread brings up something that I've thought about before. Many questions, including most of the ones here, have been asked and answered several times on this forum. If the regulars here could get together and create a cooperative FAQ, maybe Jermey would post it somewhere and we could reference it instead of having to write out answers all the time. Anyone want to help out?

[survey tech]

With respect to the use of black versus brown on topo maps, black is used for what are called hard elevations, meaning an elevation on a solid permanent object, usually noted with an x for a monument or other small object and without an x for larger objects, like paved road intersections, while brown is used for what are called soft elevations, meaning an elevation on plain ground like dirt, grass or sand. Soft elevations are usually calculated by interpolation, such as contour lines, rather than being precisely determined by direct observation and are therefore not precisely repeatable or recoverable. Brown is also used for unchecked or approximate elevation data, such as aerial photography that has not yet been verified on the ground, and is, for that reason, sometimes the prevailing color on maps in areas where ongoing revisions are being made, known as provisional maps, along with purple, which represents new structures. Provisional maps are fairly common in sparsely developed areas, such as Alaska and the Inter Mountain West, but relatively rare elsewhere. For those not already familiar with the symbols, an x is used for elevations, a triangle is used for geodetic control points and a square is used for boundary monuments

[Team Italian Greyhound]

I reported a benchmark as being "destroyed" because it was actually stated as being the "...apex of the roof of the glass-enclosed cab."

This tower doesn't exist anymore. Wouldn't it be considered destroyed?

 

POC

 

Patrick

Team Italian Greyhound

N9PRY