NGS measurements: Measured from nearest surface or center?

[TillaMurphs]

In recovery descriptions there is something we have always puzzled over. We thought we would eventually run into a clarification; however, it hasn’t happened yet.

 

In recovery logs we see measurements from trees, telephone poles and tanks but they never specify if the measurement is from the center of the object or from the nearest edge of the object. Is there a standard assumption for this? How should we interpret NGS recovery or monumentation reports when we run across distances given from cylindrical objects where it does not specify center or side?

 

Over short distances and/or with a large diameter object this can make a significant difference. Since we were unsure if there was a standard, in our reports we have started clarifying by stating “from the nearest side” or “from the center”.

 

I guess we have also wondered the same thing in regards to short distances from the witness post to disk. Is it from the nearest edge of the post to the nearest edge of the disk, or center to center, or?

 

I hope everyone had a good weekend.

 

Just pondering,

 

--The TillaMurphs

[Bill93]

It doesn't hurt to specify.

 

I've been told that anywhere it makes sense and is not otherwise noted, the standard is from the center, as for poles, trees, manhole covers, etc. But there are times when you aren't sure everybody is on the same page.

[NGS Surveyor]

I searched through several USC&GS and NGS publications and could not find the answer to your question. Here is what I did find:

 

A quote from page 120 of Special Pub. #247, Manual of Geodetic Triangulation:

 

“(6) Details of individual marks.-This section should give a short description of

the station mark, each reference mark, and the azimuth mark. The exact stamping

on each mark, the amount it projects above the surface or is covered by the ground,

and the type of mark should be stated. The detailed location of each mark should

include measurements from definite objects in the vicinity such as witness posts, center

lines of roads, fences, ditches, power poles, prominent trees, wells, houses, barns, cairns,

and shorelines. If slope distances were measured, they should be stated in this paragraph.

Horizontal measurements listed in the box section should not be repeated here.”

 

The center of roads was specified since roads are often widened and because the edge of some roads is poorly defined.

 

From NGS’ “Geodetic Operations” manual, of 1984, page 2.1-4 (not on-line):

 

“Reference distances from monuments to prominent objects should usually be measured and recorded to the nearest tenth of hundredth of a meter, depending on the distinction of the point measured to. Distances to and between reference marks must be measured to the nearest millimeter and the nearest hundredth of a foot.”

 

GeorgeL

NGS

[Difficult Run]

This is an excellent question and the answer is probably buried in an NGS publication... somewhere ...

(NGS Surveyor, we appreciate your efforts and hope you'll find a definite answer soon).

 

Here's my interpretation:

For 'solid' objects such as power poles, signs, fence posts, trees and such, I've measured from the closest edge.

Doing so eliminates the guesswork or calculation of defining a center point on so called 'solid' object.

 

When it comes to 'soft' objects, (for the lack of a better word), measuring from the center line makes more sense.

These 'soft' objects would include roads, trails, streams, manhole covers or anything you could walk across.

The only exception I can think of is a river where measurements to the center might be a wee bit difficult.

 

That's my take.

~ Mitch ~

[+Holtie22]

When I was working for the National Park Service, their standards specified putting a nail and washer in the blaze of a bearing tree to provide a more definitive point to measure to.

When I was working for the National Forest Service, their standards specified measurements made to the center of a bearing tree.

Fast forward 20 years...

If the bearing tree is still alive, the nail and washer will be covered over by new growth, but still very close to their original position. The center of the tree is also very close to where it was.

If the tree has died, (or been felled) the position of the nail and washer is eventually lost. Evidence of the center of the tree, however, (stump, stump hole, tap root, etc.) will exist for much longer.

When I locate these types of objects, (trees, utility poles, fence posts, manhole covers, etc.) for a survey, I always measure to the center of the object.

All that being said, of course, the contrary can always be shown.

[mloser]

Up until this thread I was POSITIVE the distance was to center unless otherwise specified. Why I was so sure escapes me, but I have always measured to the center of any item that didn't specify otherwise and have had good luck with it. Holtie's mention of tree centers staying fairly constant is a good one too.

[NGS Surveyor]

I sent emails to 3 retired friends of mine who all worked on NGS field parties. No one remembered a specification stating "nearest point" or "center of...". One stated that the person writing the description should always state what they are measuring from, and another found a CO-OPS (NOAA tide gauge office) spec. that instructs one to measure to the center. As I stated earlier, I could not find the answer in the several USC&GS and NGS specs. I looked in.

 

GeorgeL

NGS

[+GeoGeeBee]

Please forgive this dumb noob question, but how does one measure to the center of a tree? I can't figure out how to put the end of my tape there.

[NGS Surveyor]

Measure the circumference. Divide by Pi. Divide by 2 to get the radius. Then drill a hole exactly that depth and in-line with the direction to the other mark. Use a long screwdriver to hold the end of the tape at the bottom of the hole.

 

OR

 

Measure from the side of the tree.

 

GeorgeL

NGS

[+jwahl]

BLM has measured the distance to it's bearing trees, set on PLSS corners to the center for many years. The procedure is to hold your tape on the side of the tree at a perceived right angle to the line from the tree to the monument. There is some estimation error involved there, but not much unless it is a very large tree.

 

I suspect that is how C&GS would have done it too. A slightly more precise method could be to measure to the face of the tree and then using the circumference of the tree, determine it's diameter and radius and add that together. There is likely to be some uncertainty in any case because of where on the tree you chose to make the measurement.

 

Also remember the measurement is usually assumed to be horizontal.

[NGS Surveyor]

I was, of course, kidding when I said to drill a hole in the tree. When a measurement to the center of a tree or pole was desired, we measured to the side as jwahl also described.

 

When traversing to a large diameter tank, we would tape to the edge, then measure the circumference, compute the radius, and add the radius to the distance measured to the edge of the tank.

 

GeorgeL]

NGS

[+Black Dog Trackers]

I had always assumed that reference mark distances to power poles was to the nearest edge of the pole and a distance to a tree was to the center of the tree. I assumed this because:

1. it wasn't spelled out, and
   
2. a tree will grow, whereas a power pole will not.
   

With a distance to the center of a tree there's always going to be some uncertainty. We've all seen the rings on a cut tree and the center is not always in the center of the tree, especially in the case of hardwood trees.

 

I have measured to trees and have assumed that measuring to the "the side of the tree at a perceived right angle to the line from the tree to the monument" as jwahl says would be sufficiently accurate to find the mark.

 

I just did a calculation assuming a 30 foot distance to a reference tree that is 18" in diameter. Measuring to the side of the tree as described above gives a hypotenuse distance of just under 30.01 feet. The difference of 1/100 inch between that and 30 feet is 10x as precise as a feet-and-tenths measurement, and certainly precise enough for finding a disk mounted in a 10" or 12" diameter monumentation. It's probably precise enough for establishing the reference too.

 

So, I'd say that mathematically, doing a measurement to the side of a tree at the tangent point is sufficiently accurate for either finding or establisihing a reference feet-and-tenths distance to or from a tree.

[+pgrig]

I too measure to the outer surface of reference marks. (Often with things 30-50 ft. distant, I'm using a laser rangefinder, so that sort of requires me to read the surface. When it's a tree, I usually say something like "12.6 ft. to the bark of a 12-in. oak tree," in the hope that that will assist someone who comes back 30 years after me. I believe I have seen a lot of inconsistency on things like power poles and fire hydrants--some folks seem to measure to the center, and some to the surface. And when it's one of those giant light pole bases you see near shopping centers, it make a BIG difference...(like 15-20 in.).

-Paul

[+Klemmer]

Another opinion / experience:

I always assumed those type measurements were to the center of the object. Not sure why, just seemed logical. It hasn't ever made a difference in any recovery for me, that I can recall. In the future, I'll be more specific if new measurements to an object are needed.

-- Klemmer

[+m&h]

AJ4075, 31 D, on Nantucket Island, was something like our ninth find ever. We verified the distance from the base of a flagpole to the center of the disk, and found that it checked out from the flagpole's surface. We're sorry to confess that as time has gone on, we have been less careful to check those distances in many cases.

[+DesertDweller]

Not to hijack this thread, but it is a related question. Some datasheet describe a mark as being x miles south of some city, town, or other political unit. Are those always road or air miles and measured from where, the edge of the city, town, etc. or from the center?

[2oldfarts (the rockhounders)]

Not to hijack this thread, but it is a related question. Some datasheet describe a mark as being x miles south of some city, town, or other political unit. Are those always road or air miles and measured from where, the edge of the city, town, etc. or from the center?

 

Welcome to Benchmark Hunting.

 

We have found that they are road miles measured from the original post office of the city, town etc. unless otherwise specified.

 

Good hunting.

 

Shirley~

[+DesertDweller]

Not to hijack this thread, but it is a related question. Some datasheet describe a mark as being x miles south of some city, town, or other political unit. Are those always road or air miles and measured from where, the edge of the city, town, etc. or from the center?

 

Welcome to Benchmark Hunting.

 

We have found that they are road miles measured from the original post office of the city, town etc. unless otherwise specified.

 

Good hunting.

 

Shirley~

 

Thanks for the help. I have found a small number but hope to try for some more this winter.

[NGS Surveyor]

DesertDweller and Shirley, I did some looking on this question of air miles versus road miles. USC&GS Special Publication No. 247 doesn’t specify (see pages 119-120), but the contracting document that I wrote a few years ago reads:

 

“2.1 FIRST PARAGRAPH - The first paragraph is the description of locality. This part of the description begins by referring to the airline distance and direction (cardinal or inter-cardinal point of the compass) from the three nearest well-known mapped geographic feature(s), usually the nearest cities or towns. Use three references equally spaced around the horizon, if possible.

In writing the Description, always progress from the farthest to the nearest reference point.

Distances in this part of the description shall be in kilometers (followed by miles), or meters (followed by feet), all distances to one decimal place. Detailed measurements which appear elsewhere in the description should not be repeated in this paragraph. Points of the compass should be fully spelled out. Do not use bearings or azimuths. State the name, address, and phone number of public sector property owners (however, phone numbers of private property owners are NOT included). State any advance notice and security access requirements for reaching the station. Also state any unusual transportation methods that may be required to reach the station.”

 

My thinking is, that the main purpose of the distances from the nearest towns is to give the user some references for quickly determining the mark’s approximate position on a map. Remember, most of these descriptions were written long before on-line maps and GPS were available. Also, if a description gives a “cross-country” distance, like “8 miles southeast of Jones Mountain,” that would also be “air” miles. Once the description distances are clearly following roads, they become road distances. If there is a road due north (or east, etc.) of a town that leads right to the station, then the air miles and road miles would be the same, perhaps explaining why road miles have seemed to fit in some cases.

 

GeorgeL

NGS

[NGS Surveyor]

I just found out that program WINDESC (used to write station descriptions) computes airline miles from the 3 nearest towns.

 

GeorgeL

NGS

[2oldfarts (the rockhounders)]

What we have found is that with the old lines from the 1930's and older, the measurements are road miles. When on some of the winding roads that we have been on, air miles would have been way off.

 

We have used the given mileage to determine where the next benchmark should be located by using a found benchmark and comparing the given distance of the that mark and the mark we will look for next. Then it is a simple thing to do. Start at the found benchmark, reset the odometer and head for the next mark. It has worked well for us, don't know how it will work for you flatlanders though.

 

John

[NGS Surveyor]

Rockhounders, Your comment lit a light bulb here. We should have been discussing two different things. Triangulation station descriptions used to be quite different from vertical control station descriptions. I have been discussing triangulation station descriptions and your comment is about vertical control. See the following quote from NOAA Manual NOS NGS 3, Geodetic Leveling, page 2-40 (on-line at: http://www.ngs.noaa.gov/PUBS_LIB/Geodeticleveling_nos_3.pdf ):

 

“The distance from the nearest city or town, which is to be entered on line *18*, is not a straight-line measurement. Instead, it is the total of the distances given in the instructions to reach the point, by following the most direct highway routes.”

 

GeorgeL

NGS

[TillaMurphs]

I just found out that program WINDESC (used to write station descriptions) computes airline miles from the 3 nearest towns.

 

Is WINDESC available to the public?

 

Thanks,

[+2Jeeps2Jacks]

I just found out that program WINDESC (used to write station descriptions) computes airline miles from the 3 nearest towns.

 

Is WINDESC available to the public?

 

Thanks,

 

Right here:

 

http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml#WinDesc

 

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