2 Benchmarks same coordinates

[+catdad]

I have found two Benchmarks listed with the same coordinates N 37° 43.833 W 088° 55.550. They are HB0501 and HB0502. They are not at the same point. I confirmed this by finding both and checking the designation numbers. HB0502 is at the posted coordinates and is marked K 120. By following the description I found HB0501 at 37*43.887N 088*55.632W. I confimed it by the designation number H 120, and took a picture with my GPS. I hope this is the right place to post this and that it helps.

[+m&h]

catdad--

 

You might want to take a look at the Me First! thread in Benchmark Hunting. The #7 post there takes up the difference between scaled coordinates and adjusted coordinates. Briefly, it is not at all uncommon to find two marks with the same scaled coordinates. That is, the coordinates are in degrees, minutes, and whole seconds, and have been arrived at by applying a straightedge and a scale to a topographic map. They can be off by hundreds of yards.

 

NGS has posted notices that handheld refinements of scaled coordinates are welcome additions to the recovery note, but what you have turned up is not in the usual sense an error.

 

Good hunting!

[royswkr]

Sounds to me like they may have been running a line of survey markers and somebody entered the same coordinates twice. You can report the correction to NOAA at http://www.ngs.noaa.gov/FORMS_PROCESSING-c...y_entry_www.prl and also log it along with the BM.

[+m&h]

These two marks seem to be a little more than 500 feet apart. They were set in 1935, so the coordinates would probably not have been established instrumentally on the ground. In the office at some later time, a map would have been used, with the descriptions, to establish coordinates to the nearest second. The results were somewhat approximate. Many years later, the NGS was able to retrieve the measurements made to various marks, and to make least-squares adjustments of them. The results are the adjusted coordinates, with seconds given to five decimal places. The information on which such adjustment is based is not always available, so many data sheets retain the scaled coordinates. Since the advent of GPS, handheld receivers are often able to provide coordinates of greater accuracy than the scaled ones, and the NGS has expressed interest in having those reported. But these are refinements of accuracy, not corrections; no one has made a blunder that is now being set right.

[+seventhings]

catdad -

 

While this situation (two marks with identical SCALED) coordinates is not very common, it is perfectly explainable. SCALED coordinates MAY be up to 600 feet from the actual position of the mark, though, in my experience, SCALED coordinates are more typically 100 - 150 feet from actual.

 

It is possible, for example, for the scaling process to yield an error of 600 feet west of actual for Mark A, and an error of 600 feet east of actual for Mark B. Theoretically, then, these two marks could be 1,199 feet apart and still share the same SCALED coordinates.

 

My example is extreme, and, in my experience I've never seen two marks 1,199 feet apart that had the same coordinates. I have seen, however, marks 250 feet apart that had the same.

 

If m&h's observation is correct ("...seem to be a little more than 500 feet apart."), I would say "wow, more than 500 feet apart and with the coords", but I wouldn't say it very loudly.

 

The way to solve this mystery/limitation/problem is to take a good fix for each mark with your handheld, and include that more accurate position in your log for each mark. Furthermore, you could convert handheld reading to the NGS' DD-MM-SS.S format and include your handheld position if and when you report your recoveries to the NGS site.

 

Good hunting!

Will

[+m&h]

Our observation was pretty cursory the first time around. We just marked the two sets of coordinates as provided by catdad on Google Earth and measured the distance between them with the GE ruler. Got about 505 feet.

 

Proceeding with a touch less recklessness, we converted the coordinates to DDD MM SS.SSS, and entered them in INVERSE. For HB0501 we first used the data sheet coordinates, and then a simple decimal conversion of catdad's coordinates. Now, 60 X .833 is 49.98, but 50/60 is .8333333333333 etc. That small difference resulted in a small increase in distance using simple arithmetic conversion of catdad's decimal minutes: 156.1084 meters vs. 156.5013 meters.

[+catdad]

M&H, Thanks for the the replies. I did read the "Me first" thread, but I am new to Benchmarks and didn't quite understand. You and the others have gave me a better understanding now. I am a Army vet, so this Degrees and Minutes is also new to me. We used grid coordinates. I am learning though. You can teach an old dog new tricks, it just takes a while to sink in. Thanks again.