Bad Coordinates

[dayver]

Hi, I'm pretty new to all this, been geo-cache-ing for a while, but new to benchmark hunting.

Anyway, do you guys find that the coordinates given are a bit 'off'? Like 25 miles or so?

Or is it just me??

[+DustyJacket]

How are you getting the coordinates? Downloading them or entering by hand?

 

Make sure the datum on your GPS is the same as the coordinates you enter.

 

Even if you are entering NAD27 coordinates in a WGS84 unit, they should not be off by 25 miles.

 

DustyJacket

...If life was fair, a banana split would cure cancer.

[+Kewaneh & Shark]

Something to keep in mind when searching for a benchmark is that while a cache was placed by someone using a recreational GPSr to obtain a Lat/Long on it, the coordinates on a benchmark were not always obtained with such precision. Many benchmarks received a coordinate with a method called 'scaling'. In a nutshell, the coordinates were scaled from a map. (There's more to it than that, but for this discussion, that's enough said.) Scaled coodinates can be off quite a bit, however they are usually within one-tenth (0.1) of a mile. I've only found one that was more than that at 0.21 miles off. Usually they are within a few hundred feet.

 

Some benchmarks have an adjusted coordinate. These are more accurate, but still not always right on. It depends on how they were adjusted. The adjusted benchmarks have usually been measured to in some way using a conventional method of surveying, sometimes with GPS, and then, through mathmatical calculations, the coordinates were 'adjusted' to a more true position. These can still be off a bit. Measurement methods in surveying and datum differences between NAD27 and WGS84 and can play a part in the differences. I found one benchmark that was occupied by the CDOT in a statewide GPS network densification project and my GPSr actually zeroed out. (That was cool.) Usually they are within 100 feet or so.

 

If you are finding coordinates to be 25 miles off, something is wrong with your coordinate. Like Dustyjacket said, it could be how the coordinate is being put into your GPSr, but probably not the datum. I usually don't bother to switch the datums from WGS84 to NAD27 in my GPSr to look for a benchmark because the difference is, in many cases, negligable.

 

But that's just my take on it all...

 

When you're looking for a benchmark, use the coordinate to get you close to the location, and then use the surveyors' descriptions on the benchmark page or the original data sheets to find it.

 

Keep on Caching!

- Kewaneh

[solohiker]

I should have checked this thread before scampering off this weekend. I looked for a a couple benchmarks and came up empty.

 

I thought I had a map datum problem too. When I took a closer look at the location description in both cases my GPS had me on the wrong side of the road. The descriptions were fairly detailed so I went back to two of the locations at lunch and found both markers after a few minutes of searching.

 

The directions mentioned scaling also.

 

If these are navigational benchmarks why are then not accurately plotted? Are they just local references?

 

Where is a good place to get some history on benchmarks?

[+DustyJacket]

The NGS web site has several "history" papers.

 

DustyJacket

...If life was fair, a banana split would cure cancer.

[+Black Dog Trackers]

solohiker -

 

You're right to notice the mention of 'scaling'.

 

If the benchmark's page says "location is SCALED', under the coordinates, that is a quick clue that the benchmark is for vertical control and that its horizontal location is not going to be very accurate.

 

The ones that say 'scaled' mean that the coordinates you see were probably made by the surveyors reading a topographic map and estimating the location by map measurement, at least on some older ones. The coordinates will be up to 300 feet off (in my experience). For these, you MUST use the verbal directions instead of the coordinates, as you have found.

 

If the benchmark's page says 'location is ADJUSTED', then the coordinates will be much more accurate than your GPS can measure. Even then, pay attention to the verbal directions.

 

I recommend when starting out benchmark hunting to pick 'adjusted' ones, not 'scaled' ones to look for.