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How do I find this Benchmark?


lele2

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I came across this Benchmark while out bike riding.

 

After a little reading and searching....I believe it is a "reference mark" used to "point" the surveyor in the direction of the triangulation disk. This would make sense seeing that there is a "triangulation" disk that is recorded and found on the Benchmark pages, here: Bemis

 

Here is a picture of the one I found, sorry I just read that you are NOT supposed to put the GPS in the picture.

 

IMG_0933.jpg

 

Is this a Benchmark??? Or just a Reference Mark that does not get recorded on Benchmarking?

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Well, you're pretty much correct about everything. What you actually found was the Azimuth mark for station BEMIS. The arrow should point toward the main station mark, but azimuth marks are generally located further away from the main station than reference marks.

 

You can post a note with your photo on the Geocaching page for Bemis, ME3147, but it's not a "Found It" because you didn't find the main station disk. Occasionally, a reference mark or azimuth mark will have its own PID and you can log it, but that is not the case with this one.

 

If you look through the logs on the GC web page you linked, Team Fawlty has good pictures of the actual station mark, which will have a triangle rather than an arrow in the center, the azimuth mark that you found, and one of the reference marks. Another good photo is the view from the azimuth mark toward the main station.

 

You can load the coordinates from the Geocaching page into your GPS and should be able to pretty much walk right up to the mark. This station has "adjusted" coordinates, which means they're very accurate. Many benchmarks (most) have scaled coordinates which are not very exact.

 

If you visit the station again, be sure to take more photos and log it here on GC. While you're there, it looks like there are three reference marks in the vicinity of the main station that you could also search for and photograph. Many of us like to find all the disks associated with a triangulation station when logging it. The current NGS data sheet can be found HERE. If you look in the "box score," you'll see all three reference marks listed with their distance in meters and azimuth from the main station mark.

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lele2 -

 

Regarding reference marks versus azimuth marks, the way to distinguish them is to see what's written on the rim (usually the rim) of the disk. This one says "azimuth mark". They have different purposes; the reference mark is to help a surveyor find the main station or even to re-set it from them if it's destroyed. Reference marks are usually within 20 feet or so of the mark, usually with different but not opposite directions from the mark to give a good 'fix' on the mark's position. As andylphoto says, azimuth marks are farther away, sometimes over 1,000 feet, to provide a precise sighiting - 20 feet would be too close. An azimuth mark is to establish a direction vector based on true North (as opposed to magnetic North). With an azimuth mark vector available, surveyors can survey from the main station without having to do the work of establishing the direction to true North; they can just set up on the triangulation station and turn from a sighting of the azimuth mark.

 

There's no absolute rule to not include the GPSr in the closeup picture - it's just a preference that some of us have. In this case, we could see where you were talking about by reading the GPSr. :anitongue:

 

If you choose to log (not found as andylphoto says, unless you go back and find the triangulation station) ME3147, be sure to include in your log text the coordinates of the Azimuth mark from your GPSr. Note that there is no need to record your GPSr's coordinates for the triangulation station - it's coordinates are already exact enough.

 

Judging by the number of reference points, this is an important mark and I notice that all the local measurements are to trees and roads in 1971, 37 years ago! It would be good for one of us, perhaps you, to do a bit of re-measuring to the roads and trees for 2008. ;) Measuring from witness posts is helpful but they don't last as long as trees or even road centerlines, so they are not as good. As non-professional surveyors, I don't recommend measuring from road centerlines, but meausrements from trees, curb faces, fireplugs, etc. is generally not dangerous.

 

It would be interesting to know what the 2004 log meant by "Found it (and straightened it out.)". Manually fiddling with even a millimeter of position of a mark is bad. Finding that a mark does wiggle should be noted in the log, but no attempt should be made to 'fix' it.

Edited by Black Dog Trackers
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It's my guess the first GC log means the person found the witness post, which others describe as being in poor condition, and fiddled with it, never finding the disk. This person has 3 benchmark logs on GC, none of them clearcut winners.

 

In the pictures the disk looks pretty solid.

 

One of Coyote's pictures:

bd09aa67-c259-4a54-9533-35bc4b14ea9a.jpg

Edited by Bill93
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A witness post is an upright marker placed near a benchmark. The picture posted by Bill93 is an example. That one is a typical kind called a Carsonite witness post. They are orange, fibrous plastic and have a white sign (sticker) on the top that says there is a survey marker nearby. Witness posts are installed to help surveyors see where a benchmark is.

 

The datasheet should say where the benchmark is in relation to the witness post, like "1 foot South of a witness post". Sometimes they don't, unfortunately.

 

There seems to be no standard as to how far the witness post is from the benchmark or how it is oriented with respect to the benchmark's position. Sometimes they are 1 foot away, sometimes they are 20 feet away, sometimes the mark is in front of the post, sometimes behind it.

 

In some areas, witness posts are cement monuments about 4" square and several inches high and say B.M. or something similar. Other witness posts are 5 foot tall steel stakes like those used in gardening and a small rectangular metal sign like a license plate is mounted vertically at the top.

 

The position of the witness post has no geodetic value whatsoever. It only indicates that there is a benchmark near it and hopefully the datasheet says how to find the benchmark from the witness post.

 

Several witness posts I have found have been knocked down by mowers or whatever and not in their originial position.

 

Here are some examples on Dustyjacket's site now hosted by BuckBrooke.

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I was under the impression that the witness post recorded the place of an acutal human witness who observed the setting of the mark. :anitongue:

 

A witness post is an upright marker placed near a benchmark. The picture posted by Bill93 is an example. That one is a typical kind called a Carsonite witness post. They are orange, fibrous plastic and have a white sign (sticker) on the top that says there is a survey marker nearby. Witness posts are installed to help surveyors see where a benchmark is.

 

The datasheet should say where the benchmark is in relation to the witness post, like "1 foot South of a witness post". Sometimes they don't, unfortunately.

 

There seems to be no standard as to how far the witness post is from the benchmark or how it is oriented with respect to the benchmark's position. Sometimes they are 1 foot away, sometimes they are 20 feet away, sometimes the mark is in front of the post, sometimes behind it.

 

In some areas, witness posts are cement monuments about 4" square and several inches high and say B.M. or something similar. Other witness posts are 5 foot tall steel stakes like those used in gardening and a small rectangular metal sign like a license plate is mounted vertically at the top.

 

The position of the witness post has no geodetic value whatsoever. It only indicates that there is a benchmark near it and hopefully the datasheet says how to find the benchmark from the witness post.

 

Several witness posts I have found have been knocked down by mowers or whatever and not in their originial position.

 

Here are some examples on Dustyjacket's site now hosted by BuckBrooke.

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No they are simply as described in the previous posts, nothing to do with a 'human witness' except of course humans set the mark, the RM's the witness post and wrote up the data sheet. I guess the posts are more of the 'bearing silent witness' type.

 

I was under the impression that the witness post recorded the place of an acutal human witness who observed the setting of the mark. :anitongue:

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I don't really have anthing to add to the topic, it just caught my attention as I recognized the station name. I picked this mark up during lunch one day. I measured to all the RMs and only found RM 3. RM 1 measured to the edge of the entrance driveway and could be under the gravel. They must have set this in the winter judging by all the water tanks measured.

 

Brendan

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The datasheet should say where the benchmark is in relation to the witness post, like "1 foot South of a witness post". Sometimes they don't, unfortunately.

Or they give incorrect information. Such as this one:

 

HR2378'THE MARK IS 0.3 METERS W FROM A WITNESS POST

 

Wanna guess where it was? 30 inches northeast from the witness post, under layers of dirt and lichen. :rolleyes: I found it after spending 15-20 minutes scraping off everything within a meter of the post...

 

I don't know whether the original description was wrong, or whether NGS replaced the witness post at some point and never filed an update.

 

Patty

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I recovered RL0515 earlier this year. After arriving at the site, I searched briefly (unsuccessfully) near the witness post before closely reading the datasheet and measuring from the road & tracks. I found the destroyed benchmark exactly where it was supposed to be. The witness post had obviously been replaced, but I've got to believe that the NGS or whoever replaced the wooden post with a metal one would have probably placed it in about the same area. :blink:

b82a9f2c-95ae-4e18-8050-34b56da0a76d.jpg

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lele2 -

Judging by the number of reference points, this is an important mark and I notice that all the local measurements are to trees and roads in 1971, 37 years ago! It would be good for one of us, perhaps you, to do a bit of re-measuring to the roads and trees for 2008. :laughing: Measuring from witness posts is helpful but they don't last as long as trees or even road centerlines, so they are not as good. As non-professional surveyors, I don't recommend measuring from road centerlines, but meausrements from trees, curb faces, fireplugs, etc. is generally not dangerous.

 

FIRST ~ Thanks for all the great information about the "Azimuth" which I came across while bike riding. I have seen several marks of this sort and have not pursued them ~ and I believe "Benchmarks" too!

 

They are interesting and I would like to learn about Benchmarks ~ thanks for supplying the links to "photos" and "information" about Benchmarking.

 

Now, I have some other questions ~ I have driven by the "triangulation disc" every time I come in to the preserve. I have seen what I now know is a "witness post". I have not noticed it lately as this area saw a lot of construction this past year. I will be "physically" checking it out and photographing it the next time I go to Bemis Woods....which still has not gotten to my questions:

 

* Could you explain more what you mean by "re-measuring to the roads and trees for 2008"?

 

* What did "RL0515" have to do with this posting?

 

* One reason I have shied away from Benchmarking are the "datasheets" ~ which I know to be vitally important information to read ~ what is the most important information on the sheet?

 

* The link provided to Dustyjacket's sites are very helpful ~ are there any other sites with good explanations that someone unfamiliar with surveying to read?

 

* Why are there so many "Reference Points linked with this Benchmark?? And many of them are quite far away, why are they listed?

 

thanks again, glad this sparked lots of helpful information!

lele2

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Remeasuring: because roads get rebuilt, trees fall down and new ones grow, etc. when you find a disk it is good to report which old measurements are no longer useful and if possible substitute new ones. Then if the disk gets covered up someone will have a better chance of finding it, and if it gets removed it will save someone a lot of searching for something that isn't there.

 

RL0515: thread topic migration. We were talking about a witness post that might have been moved and someone gave an example.

 

Most important thing on the data sheet: For us, often it is the To Reach description in the recovery logs. If the mark has SCALED coordinates, they may or may not get you within sight of the disk location and the description is vital. For ADJUSTED coordinates, the GPS may get you there but if the mark is buried under dead grass and leaves the measurements in the description may make the difference between finding it and not. There are other things that are useful sometimes, such as the box score on a horizontal control mark, but I won't go into that other than to say it gives more distances and angles to help the search.

 

Reference points: These are extracted from the "box score" on the data sheet. They are points to which angles were measured to help determine the position of this mark. This is an unusual large number of reference points.

 

Always fetch the latest version of the NGS data sheet, because the geocaching data base is a fixed snapshot from several years ago. One handy way is to put the PID number in this syntax to make a link: http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=me3147

and another is to go to the NGS web pages and follow the links for Data Sheets to get several search options.

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