Coordinates Accuracy

[H to the Bizzle!]

Hi All! I'm planning on getting more active in the aspect of bench-marking.

 

I've been reading about how the coords are usually just a starting point and from there you should just read the description, which got me wondering how off the coordinates usually are?

 

Thanks for any info you might have.

[ArtMan]

All benchmarks are very accurate. (Some are more accurate than others, but we can go into that later.)

 

Some benchmarks are accurate for horizontal information, i.e. latitude and longitude. Others are accurate for vertical or elevation information. (Technically, these are the only ones that surveyors will usually term "bench marks.") A relatively small number are accurately measured for both horizontal and vertical information.

 

From the FAQ...

 

Why do the coordinates of some benchmarks seem to be way off?

Control points that are used primarily for vertical control usually don't have very accurate horizontal coordinates. Their coordinates will get you within a couple hundred feet of the marker, but you will probably need to use the description to find it, and you will definitely need to use the description to confirm the true identity and location of the point, with reference to nearby objects. The reason for this is that the surveyors that established the benchmark used rulers on a topographic paper map to determine the coordinates. (Pre-GPS) The vertical coordinate is extremely accurate, however, since it is the purpose of such a benchmark to provide a reliable true elevation, for mapping and construction projects. On the benchmark's page, look at the line under the marker's coordinates. If it says "location is SCALED", then the coordinates were probably originally obtained from a map and won't be very accurate.

 

Control points that are used for horizontal control have much more accurate coordinates. (In some cases, those coordinates can seem almost hyper-accurate.) If, on the benchmark page, it says "location is ADJUSTED", then you are looking for a horizontal control point. Most of these have been included in a nationwide network adjustment to obtain state-of-the-art precision, and the coordinates can be much more accurate than any handheld GPS can measure (within a few millimeters).

 

-ArtMan-

[Photobuff]

Not only were most of the coordinates taken off maps, which is one source of error, but (for the ones I'm looking at) the coordinates are only listed to one second. If you just add and subtract one second from each number and make those the four corners of a box, you'll have a pretty big box. If you take the disclaimer on the data sheet of plus and minus six seconds, that's an even bigger box. IMO, the only purpose of the scaled coordinates is to make sure you didn't take a wrong turn on your way to the site!

[ArtMan]

In my experience, scaled coordinates are usually much more accurate than +/- six seconds (which translates typically to a 1200' diameter area of uncertainty. In fact, more often than not you'll find the mark within 100 feet of the nominal position. To be sure, that's not walk-up-to-it accuracy, but it will certainly get you to the general area. After than, read the description, which was probably written in pre-GPS days and was intended to be sufficient to locate the mark without a GPS receiver

 

-ArtMan-

________

 

Edited to correct stupid goof — it's +/- six seconds (not minutes, as I first wrote). Thanks to Klemmer & TeddyBearMama for catching it.

Edited January 21, 2006 by ArtMan

[+seventhings]

My experience is very similar to ArtMan's. For a while, I actually kept track of the deviation between the published coordinates and the actual position as measured by my handheld. (I just noted the "GOTO" distance when I was at the mark). For a sample of about 50 marks with SCALED horizontal coordinates, I measured the average deviation at 129 feet.

 

I have also come across three marks where the deviation was 0.35 miles, 0.40 miles and 0.50 miles respectively (and two of the three were in Perth Amboy, NJ!!).

 

Interestingly, when I've found marks with SCALED coordinates that another benchmark hunter found before me (and, when that benchmark hunter listed his or her handheld's position and I've gone to the trouble of loading the other hunter's coordinates into my handheld), I've noted that the deviation between his or her handheld and mine is generally only about 15 feet on average. That deviation (7.5 feet for each hunter) is pretty good, I think, as it is about the same that I generally get between my handheld and the actual position for marks with ADJUSTED horizontal coordinates.

 

Will

Edited January 21, 2006 by seventhings

[Wintertime]

For a sample of about 50 marks with SCALED horizontal coordinates, I measured the average deviation at 129 feet.

 

I have also come across three marks where the deviation was 0.35 miles, 0.40 miles and 0.50 miles respectively (and two of the three were in Perth Amboy, NJ!!).

Yeah, I had one at Yosemite a few months ago that was off by .25 miles. Just what I wanted at 8,000 foot elevation--another half mile of walking. ;-)

 

Patty