Technical Puttering & Wondering

[+m&h]

We were playing around with the data on a recent find. It is CITY (EP0236) in Vaughn, New Mexico. It's a triangulation station, so it is equipped with two reference marks. All three disks are in excellent condition. As sometimes happens, each reference mark has its own PID (EP0238 for RM 1, and EP0237 for RM 2 -- odd reversed numbering, but there it is) and data sheet. A fat find for the score-minded, we'd guess. For us it was just one of those heart-leaps-up moments, on a low ridge above a seemingly dying town, in bright sun and strong cold wind. There they were, where they said they'd be. Anyway . . . .

 

The coordinates on the primary mark are adjusted. Those for the reference marks are scaled. But the "box score" on the primary mark gives azimuth and distance to each of them, so why not use the program FORWARD.EXE (from the NGS toolkit) to refine the coordinates? One reason in this instance, we suppose, is that the azimuths are given only to the nearest minute. But what about this? Is FORWARD.EXE sound enough for that sort of job?

 

Thanks for your thoughts.

[+PFF]

I recently began using the program and it has produced good results. Most often, I use it for predicting where the reference mark can be found when no coordinates are given. Unfortunately, one of the components (either bearing or distance) frequently is not given--or is approximated.

 

I encourage others to give this program a try. Over time, we can compare notes about how to use it under various circumstances. By the way, it's not just for reference marks. Sometimes a target is described in terms of distance and bearing from another station. The program can help with this situation, if the referenced station is adjusted (or if you already have found it and taken a GPS reading).

 

-Paul-

[itchytweed]

Over in Benchmark Hunting, I posted a FAQ offering on how to do just this but not using FORWARD.EXE (windoze only) but a spreadsheet with Vicenty's Equations. I also include a blurb on how to do the routines. I prefer the spreadsheet option because I can tweak if necessary.

 

Take a look.

[+PFF]

This is a follow-up to the question of whether the FORWARD program could have value in our hobby. A recent experiment clearly shows that it would.

 

In the Benchmarking forum, Kit Fox asked for help finding a mark at the old airbase where he works. The coordinates were scaled, and the description said it was located 225 feet north of the door on an airplane hanger. I used FORWARD to generate a set of coordinates, which Kit Fox took to the site. It gave him the starting point for locating the benchmark. He then used his handheld GPS receiver to take a reading.

 

Here are the results:

 

34 42.181 PREDICTED LATITUDE

34 42.177 HANDHELD GPS INDICATION

 

118 14.052 PREDICTED LONGITUDE

118 14.049 HANDHELD GPS INDICATION

 

In this case, the starting point was approximate, being based upon an aerial photograph with a resolution of 1:10,000. Plus, it was not known exactly where on the building the southwest door was located. Still, the results proved to be acceptable, and represented a considerable improvement over the scaled coordinates on the data sheet.

 

(The scaled coordinates are N32 14.133, W118 14.033, which is an error of nearly 300 feet).

 

-Paul-

 

________________________________________

As mentioned above, the FORWARD program

can be found in the Toolbox area of the NGS web

site. It will predict a set of cordinates, based upon

a starting position, plus the distance and bearing

to the target.

________________________________________

[wister6813]

Thanks for the heads-up on FORWARD.EXE. Until the mention in this forum I was unaware it existed.

 

Since the first of the year I've been using the program to generate the coordinates for the reference and azimuth marks for triangulation stations. The coordinates generated for reference marks have been right on. The coordinates for the azimuth marks aren't as accurate but that is due to the inaccuracy of the distance obtained from the datasheet description, generally expressed in tenths of a mile.

 

I was beginning to hate tri-stations because of the need for a compass, tape measure, ect. Having the point in the GPSr to show the way is much easier.

 

Bob

[Z15]

There is no way to adjust those marks. They are opened ended marks, meaning only observed from the primary station and not to the same precision as the main mark.

 

This can get into a huge discussion on whys and why nots.

 

It is what it is.

Edited February 12, 2006 by Z15

[Bill93]

>There is no way to adjust those marks.

 

I'm not sure which comment you are replying to. As you point out the box score info is for short distances so the angles aren't as accurate, and also distances (particularly az mark distance) aren't as accurate as those between triangulation stations.

 

But the values you calculate are still probably as accurate than the amateur hunter's compass and tape measurements. For nearby ref marks, the coordinates you get from FORWARD will be essentially as good for use on the handheld as adjusted marks, even though they aren't geodetic quality. The limitation for us is the ability to determine our handheld's position, not the accuracy of the coordinates (except az distance).

 

I would think that it is usually worth trying the FORWARD coordinates since that is so easy. Then if you have found one disk as a starting point, taping will probably give an improved position for others, so you can go ahead and do that. If you haven't found any of the disks yet, then the FORWARD coordinates may be a big help in getting started.

[Z15]

I mean they are not accurate for NGS to list on the datasheet using that program. I thought that was one of the questions that was asked.

Edited February 14, 2006 by Z15

[+m&h]

I mean they are not accurate for NGS to list on the datasheet using that program. I thought that was one of the questions that was asked.

We see why you thought that! Our question about the accuracy of the procedure wasn't to imply that the coordinates thus computed might meet the NGS datasheet standards. As we understand it, you can’t pass the same order of accuracy from one single mark to another. Moreover, adjusted coordinates are adjusted from a number of coordinate pairs. You can’t do a least squares adjustment on one pair of coordinates.

[+Crystal Sound]

Just to throw my $02 in...

 

I was involved over the weekend in helping to locate a property corner in some woodland. Without going into alot of details, I use FORWARD to project the property corners from the "known" starting point.

 

Even with using ~20ft error starting coordinates, and among trees in a valley for an end point, I managed to find a couple paint marked trees not very far from where the gps led me.

 

Did I find it useful? Yes. The forward function on my GPS only works to a degree & .1 mile increments. The FORWARD program gets much finer with it's decimal places.

 

I could see where it would have been some help last fall when recovering an azimuth marker.