Coordinate accuracy?

[+Mr Kaswa]

Hello,

 

I have converted a few data sheets into gpx files by copying into a txt file and converting with NPS>>GPX and then loading them into GSAK. Compared to gc.com's coordinates the converted data sheets would appear to be more likely to be accurate. Is this true, or can they be just as far out as gc.coms?

thanks,

Mr Kaswa

[+LSUFan]

Hello,

 

I have converted a few data sheets into gpx files by copying into a txt file and converting with NPS>>GPX and then loading them into GSAK. Compared to gc.com's coordinates the converted data sheets would appear to be more likely to be accurate. Is this true, or can they be just as far out as gc.coms?

thanks,

Mr Kaswa

 

The first thing you need to understand about benchmarks is if the coordinates listed on the datasheets are "adjusted" or "scaled". Adjusted coordinates will be precise, even more so than your handheld gps unit. Scaled coordinates can be off as much as 600+ feet.

 

Here is some reading on that subject.

 

http://www.geocaching.com/mark/default.aspx#adjscl

 

http://www.nelageo.net/index.php/forum/13-benchmarking-and-related-topics/10956-understanding-a-ngs-datasheet-scaled-vs-adjusted-coordinates

Edited October 7, 2011 by LSUFan

[+EdrickV]

The coordinates on the Geocaching.com pages for benchmarks would come from the datasheets from 2000, would would be as accurate as those datasheets were. If the coordinates have been changed since 2000, then obviously the coordinates will be off.

 

The bigger factor is whether the coordinates are Adjusted, Scaled, HH_GPS, or NO CHECK.

 

HH_GPS = Handheld GPS coordinates. So accuracy may be similar to Geocaches, with the combined error factors of the two GPS units.

 

NO CHECK = Don't know for sure, but these sound like they may be less accurate, or as accurate, as Scaled coordinates. The datasheets for these that I've seen all say "The horizontal coordinates were established by classical geodetic methods and adjusted by the National Geodetic Survey in February 1997. No horizontal observational check was made to the station." That would seem to indicate that the NGS does not actually know how accurate the coordinates are.

[+Mr Kaswa]

Thanks for the replies.

I understand that even with the coordinates I could be begining the search several hundred feet off based on scaled coordinates, that's not a problem, and now that it has been brought to my attention I see the notes for hand held in some sheets. I was wondering if perhaps an error had been introduced during the conversion to the gpx file, but looking into it further I see now the subtle difference between the recent datasheet and the one that gc.com displays, and I must admit that I did not expect there would be any difference in the last 11 years on a location that has been underwater for the last 70+ years. The answer would seem to be that I should go with the recent sheets over gc.com, both may scaled coords but the older set would seem to be an extra couple hundred feet off.

Mr Kaswa

[Wintertime]

I must admit that I did not expect there would be any difference in the last 11 years on a location that has been underwater for the last 70+ years.

Does this mean you're getting out the SCUBA equipment to try to recover some benchmarks? :-) There's a lake near a friend's house that was formed when a dam was built, but as far as I know, no one has ever attempted to find the benchmarks that are now under it. I wonder whether any of them would still be findable or whether sediment deposited from the incoming river would have covered them all.

 

By the way, it looks like you're in BC, and I don't know whether the Canadian government offers topographic maps that include survey marks, but here in the States, one can often get more accurate coordinates by looking for marks on USGS topo maps. I've done that for all the 7.5' quads in Yosemite National Park, and have had very good success getting very accurate coordinates for marks whose scaled coords on the NGS data sheets can be off by hundreds of feet or more.

 

Patty

[+Mr Kaswa]

I must admit that I did not expect there would be any difference in the last 11 years on a location that has been underwater for the last 70+ years.

Does this mean you're getting out the SCUBA equipment to try to recover some benchmarks? :-) There's a lake near a friend's house that was formed when a dam was built, but as far as I know, no one has ever attempted to find the benchmarks that are now under it. I wonder whether any of them would still be findable or whether sediment deposited from the incoming river would have covered them all.

 

By the way, it looks like you're in BC, and I don't know whether the Canadian government offers topographic maps that include survey marks, but here in the States, one can often get more accurate coordinates by looking for marks on USGS topo maps. I've done that for all the 7.5' quads in Yosemite National Park, and have had very good success getting very accurate coordinates for marks whose scaled coords on the NGS data sheets can be off by hundreds of feet or more.

 

Patty

 

LOL The lake behind a dam part is right, but no scuba diving for me, can't even swim but the lake does have lower than normal water levels this year. I'm going to be in the area anyway and it seemed an interesting idea to possibly find one of them, but it doesn't look promising. It looks like the benchmark will still be about 7 feet under, as well, the things used to describe it's location will be either deeper or were torn down before the flooding.

 

Mr Kaswa

[Wintertime]

(unneeded quotage deleted)

 

LOL The lake behind a dam part is right, but no scuba diving for me, can't even swim but the lake does have lower than normal water levels this year.

That friend and I thought about looking for un-submerged benchmarks at his lake a couple of years ago when the water levels were way down, but we didn't. Maybe next time--although we'd have to find places where we can get out of the boat and tie it up while we look for the marks, which could be tricky.