is NAD27 useful for old marks?

[krazykatzen]

I'd like to try finding more benchmarks, but today I was batting 1/4. The only one I found today (HL0299) was because the topo map made it easy. The SCALED coordinates had an error of +/- 6 seconds. I think each second is about 31 meters horizontal distance, so this is a HUGE search grid in rough terrain! I think the actual coordinates were closer to +/- 2 seconds. Since the mark was set in 1934, I wondered if I would be better off trying to use Datum NAD 27 to project the coordinates on my GPSr, or is the location description still my best bet? (note: many marks reference structures that have been gone over 50 years, so this strategy has limitations too).

 

Any advice?

 

Bonus questions:

 

1. aside from NGS benchmarks, is there any benefit to tracking other benchmarks I stumble across. Some are located in areas not open to the general public, so I am hesitant to post them on Waymarking.org, but they are nearly 100 years old in remote areas, so I figure it might be useful knowledge to someone.

 

2. are photos of the mark in relation to modern landmarks useful?

 

Thanks!

[+LSUFan]

The SCALED coordinates had an error of +/- 6 seconds. I think each second is about 31 meters horizontal distance, so this is a HUGE search grid in rough terrain! I think the actual coordinates were closer to +/- 2 seconds. Since the mark was set in 1934, I wondered if I would be better off trying to use Datum NAD 27 to project the coordinates on my GPSr, or is the location description still my best bet? (note: many marks reference structures that have been gone over 50 years, so this strategy has limitations too).

 

2. are photos of the mark in relation to modern landmarks useful?

 

Thanks!

 

Hi krazykatzen. Yes, most scaled marks have a +/- of six seconds. You are better off using the description for scaled marks, and it can be tough if the reference stuff has been removed or changed. I wouldn't think changing the datum would help, as the numbers you are currently using, are ones that usually came from someone taking a ruler and topo map to arrive at them for the scaled marks. They used the current datum, so if it's wrong with it now, it would be wrong with NAD27. Adjusted marks should already have been converted from NAD27 to the current datum.

 

You do want to take gps readings with your handheld unit when you find scaled marks and submit them. Your gps numbers are going to be a lot closer than +/- 6 seconds. You do want to note new reference marks if the old ones are gone, as well as take pics of the mark, and things in relation to the mark (roads, houses, big trees, and other more permanent things) to aid in future recoveries.

 

The following is taken from DSWorld's help file, in showing what the NGS would like from ones submitting recoveries:

 

Field Procedure

 

To recover a geodetic control station, follow the the 'to reach' narrative of the description to reach the area of the mark. Next, measure the distances from nearby reference objects as stated in the description. Sometimes the mark may be covered by dirt, leaves, or debris. Some marks may be buried as much as 2 ft below the surface of the ground if the mark is in a plowed field.

 

Once the mark is found, check to see if the published description is adequate for future recovery. If not, make new measurements to nearby reference objects.

 

Good reference objects are house corners, utility poles, roadway centerlines, fences, unique trees, headstones, property markers, drainage pipes, ditches, bridges and other permanent structures. Use at least five reference objects if possible. Avoid trees that are not unique or are likely to be cut for timber. Avoid using more that one pole from the same utility (the utility company may move or remove all of their poles at the same time). When using corners of buildings, bridges or other large structures, state which corner is used. In all cases the center of the object is implied unless otherwise stated. The following is an example of the reference paragraph:

 

It is 50.6 feet northeast of the center of Blankenship Road, 35.2 feet Northwest of power pole number 235, 27.6 ft South-southeast of telephone pole number 2, 15.0 ft south of the headstone of Mildred E. Thompson, 3.0 ft Southwest of a witness post and set in the top of a 12-inch round concrete post flush with the ground.

 

Next, capture the latitude and longitude of the point using a hand-held GPS receiver.

 

Next take digital photographs of the point.

 

Positioning a Bench Mark with a Handheld GPS Receiver

 

General Information

Navigational grade receivers, which are typically hand-held devices, are usually accurate to about 3-5 meters under most conditions. Positions from this type of receiver should be submitted to NGS as HH2 positions and shown to 1 decimal place of a second of Latitude and Longitude. This type of receiver is considered an Autonomous GPS (AGPS) receiver, even though it receives a corrector of sorts via the WAAS signal.

 

GIS grade receivers, which are also hand-held devices, are usually accurate to less than 1 meter (3 ft) under most conditions. They receive a correction signal from the Coast Guard or other source which corrects the signal to sub-meter precision. Positions from this type of receiver should be submitted to NGS as HH1 positions and shown to 2 decimal places of a second of Latitude and Longitude. This type of receiver is considered a Differential GPS (DGPS) receiver.

 

Survey grade receivers, usually tripod or pole mounted devices, are usually accurate to less than one centimeter (0.03 ft). These too can be submitted to NGS as HH1 positions and shown to 2 decimal places. One may also submit survey grade positions to NGS to 5 decimals, provided they meet NGS requirements for that type of positioning.

 

 

HH2 and HH1 Positioning

First, check to make sure the receiver is set up for NAD83, with the display in DDD MM SS.s format. The receiver must pick up the WAAS signal or other corrector.

 

Standing to the north side of the monument, hold the GPS receiver over the monument for several seconds. Note the estimated accuracy of the position displayed. For AGPS receivers the accuracy should be less than 20 ft (6 meters). For DGPS receivers, the estimated accuracy should be less than 3 ft. (1 meter). If the accuracy exceeds this amount, tree cover or satellite configuration may be causing the problem. Revisit the point when satellite configuration is more favorable.

 

Capture the position of the point. Change the name to the PID of the mark. Change the point description to the designation of the mark and save the point.

 

When all recovered points have been positioned, export the file to a .gpx file using software supplied by the manufacturer. To submit the file to NGS, click here for more information. To submit a single position to NGS, click here for more information.

 

Photo types:

 

CLOSE-UP - A photo taken directly above the survey mark, approximately 18 inches away from the disk. Remove any equipment, dirt, debris, water, or snow to show a clear image of the complete mark. Avoid shadow lines crossing the disk. If there is a logo cap, the logo cap should be open to show the datum point. The intent of this photo is to clearly show the mark, its condition, and all stamping on the mark or logo cap so that it is clearly legible.

 

EYE-LEVEL - A photo taken directly above the survey mark from eye level to show the monument and cover an area about 1 meter in radius, all around the mark. Remove any equipment, dirt, debris, water, or snow from the mark to clearly show the disk and the setting. If it has a logo cap, the logo cap should be open to show the datum point. The intent of this photo is to show the general condition of the mark and the immediate surrounding area, especially any condition that would be a threat to the stability or permanency of the mark.

 

HORIZONTAL - A daylight photo oriented horizontally, with the tripod or a target highlighting the mark location, and with reference objects, significant obstructions, or possible multi-path sources in view. Remember to record the direction the photo is facing.

Edited July 2, 2012 by LSUFan

[+EdrickV]

I'd like to try finding more benchmarks, but today I was batting 1/4. The only one I found today (HL0299) was because the topo map made it easy. The SCALED coordinates had an error of +/- 6 seconds. I think each second is about 31 meters horizontal distance, so this is a HUGE search grid in rough terrain! I think the actual coordinates were closer to +/- 2 seconds. Since the mark was set in 1934, I wondered if I would be better off trying to use Datum NAD 27 to project the coordinates on my GPSr, or is the location description still my best bet? (note: many marks reference structures that have been gone over 50 years, so this strategy has limitations too).

 

Any advice?

 

Bonus questions:

 

1. aside from NGS benchmarks, is there any benefit to tracking other benchmarks I stumble across. Some are located in areas not open to the general public, so I am hesitant to post them on Waymarking.org, but they are nearly 100 years old in remote areas, so I figure it might be useful knowledge to someone.

 

2. are photos of the mark in relation to modern landmarks useful?

 

Thanks!

 

As has been said, the description is likely to be far more useful then the coordinates with regard to scaled benchmarks. (Note that if you are using coordinates from Geocaching.com benchmark pages, it's possible the benchmark may have more accurate coordinates in the NGS datasheet rather then the Geocaching.com cached copy.) As far as long lost landmarks, if such landmarks are the kind that would show up on topographic maps, then you could try looking for old USGS topographic maps. They allow downloading digital copies of old topographic maps for free. There may be other old maps that you might be able to find. (I forget where I found them, but I actually was able to find a digital copy of an original plat map from the 1800's. Now that was amazing to find.)

 

As far as non-NGS marks, if a mark is on private property (and I mean like someone's home/farm/other residential type property) then I would not bother trying to make it a Waymark. If the mark is somewhere that benchmark hunters might be able to get permission to search for it, then deciding whether to Waymark it or not would be a judgement call. (Though talking to the person/people in charge would probably be a good idea.) For the most part I don't think Waymarks get the kind of attention (and traffic) that Geocaches do. Though if a specific procedure is needed to gain access to the mark, then obviously that info would need to be on the Waymark page. (I've actually seen similar things in some NGS datasheets. "Contact so and so at 555-123-4567 to get access.")

 

The one thing you could do, even if a mark is not accessible to other benchmark hunters, would be to post about it in a thread here (including pictures if available) so you can share your experience finding it. I did that for a PLSS Quarter Section Corner that I knew is on private property (that I happened to have access to) but obviously I'm not going to Waymark it.