Calculating coordinates

[+Team LaLonde]

I had trouble while hiding a difficult cache in the side of a cliff today. The cliff was on the edge of a lake in a cove so the GPSr naturally acted a bit strange. On top of that it was snowing so hard I could only see about 150 feet. I could not get a really good lock when I was on top of the cache, but the accuracy from out on the ice was good. I took several readings from both locations. I want to make sure the coordinates are as accurate as possible when submitted for the reviewer so how should I list them? Should I:

 

A. Use the coordinates straight from the GPSr at ground zero when the accuracy was 94 feet?

 

or

 

B. Use calculated coordinates from walking out into the open where the accuracy is 24 feet and work the numbers with an estimated distance and bearing?

 

Mapsource calculates the distance between the two locations (GZ and clear ice) as 528 feet when the actual distance was about 40 feet. My gut tells me to go with Option B.

[+WaldenRun]

I had trouble while hiding a difficult cache in the side of a cliff today. The cliff was on the edge of a lake in a cove so the GPSr naturally acted a bit strange. On top of that it was snowing so hard I could only see about 150 feet. I could not get a really good lock when I was on top of the cache, but the accuracy from out on the ice was good. I took several readings from both locations. I want to make sure the coordinates are as accurate as possible when submitted for the reviewer so how should I list them? Should I:

 

A. Use the coordinates straight from the GPSr at ground zero when the accuracy was 94 feet?

 

or

 

B. Use calculated coordinates from walking out into the open where the accuracy is 24 feet and work the numbers with an estimated distance and bearing?

 

Mapsource calculates the distance between the two locations (GZ and clear ice) as 528 feet when the actual distance was about 40 feet. My gut tells me to go with Option B.

 

See if you can zoom in with GoogleEarth and convince yourself that you have a good idea where the cache is in the photo. Mark a waypoint there and use the coordinates from GoogleEarth.

 

-WR

[+TrailGators]

I had trouble while hiding a difficult cache in the side of a cliff today. The cliff was on the edge of a lake in a cove so the GPSr naturally acted a bit strange. On top of that it was snowing so hard I could only see about 150 feet. I could not get a really good lock when I was on top of the cache, but the accuracy from out on the ice was good. I took several readings from both locations. I want to make sure the coordinates are as accurate as possible when submitted for the reviewer so how should I list them? Should I:

 

A. Use the coordinates straight from the GPSr at ground zero when the accuracy was 94 feet?

 

or

 

B. Use calculated coordinates from walking out into the open where the accuracy is 24 feet and work the numbers with an estimated distance and bearing?

 

Mapsource calculates the distance between the two locations (GZ and clear ice) as 528 feet when the actual distance was about 40 feet. My gut tells me to go with Option B.

Something to keep in mind is that every .001 minute is about 6 feet. So you can estimate coords from a clearing that is directly N, S, E or W of GZ by adding/subtracting this adjustment. If you are S or E of GZ then you would add .001 for each 6 feet. I use this technique when hiding a cache in tree cover, because I don't have one of those new fangled GPSs with the Sirf III chip.

[+fizzymagic]

See if you can zoom in with GoogleEarth and convince yourself that you have a good idea where the cache is in the photo. Mark a waypoint there and use the coordinates from GoogleEarth.

Not, in general, a good idea.

 

Google Earth image registration can be quite a ways off. I've seen places where it is wrong by a couple hundred feet. So you could end up with very bad coordinates this way.

 

If you can verify the validity of the image registration, however, you might be able to use this technique. To do so, you should find several prominent features within a tenth of a mile or so of the location in question. Go out and measure their positions with the GPS, and then plot those positions on Google Earth. If they all match exactly, then the registration of the image should be good enough to use; otherwise, don't use Google Earth.

 

I prefer the method proposed by the original poster: find a few points near the cache location at which you can get good coordinates and from which you can get good bearings. If you have 2 or more such positions, the cache position can be determined accurately by projecting lines at the correct bearings and locating the intersections. Since your bearing angle will likely be no more accurate than 1 degree or so, keep the points within a couple hundred feet of the cache location.

Edited February 17, 2007 by fizzymagic

[+Team LaLonde]

Thanks for all the input. I opted for Plan B. Frankly, choosing coords that I knew were not very accurate didn't seem like a responsible thing to do. Not that I'm a whiz or anything, but I trust my math and mapping skills more than I trust a GPSr with accuracy of nearly a hundred feet. I added a puzzle to the cache, making it our first 5/5.

[+StarBrand]

Thanks for all the input. I opted for Plan B. Frankly, choosing coords that I knew were not very accurate didn't seem like a responsible thing to do. Not that I'm a whiz or anything, but I trust my math and mapping skills more than I trust a GPSr with accuracy of nearly a hundred feet. I added a puzzle to the cache, making it our first 5/5.

Keep in mind that a true 5 on difficulty should be a serious challenge to find and require multiple trips to complete. A 5 on terrain is reserved for caches where special equipment is required to physically reach the cache location.

 

Most 5/5 caches are quite overrated on one scale or the other.

[+TrailGators]

Google Earth image registration can be quite a ways off. I've seen places where it is wrong by a couple hundred feet. So you could end up with very bad coordinates this way.

I've heard this but Google Maps seems to be very accurate in the San Diego area. Of course, if our GPS measurement tool is only accurate with 15-30 feet then it's hard to know if Google Maps is off unless it's way off. By the way, don't the reviewers use Google Maps to check the locations of new cache submittals?

[+WaldenRun]

Google Earth image registration can be quite a ways off. I've seen places where it is wrong by a couple hundred feet. So you could end up with very bad coordinates this way.

I've heard this but Google Maps seems to be very accurate in the San Diego area. Of course, if our GPS measurement tool is only accurate with 15-30 feet then it's hard to know if Google Maps is off unless it's way off. By the way, don't the reviewers use Google Maps to check the locations of new cache submittals?

Yes Fizzy, I certainly can understand where the GoogleEarth coords need to be checked against a second source. Can you supply a set of continental US coords where I can see a large discrepancy with something like USAPhotoMaps or Topozone?

 

Thanks,

-WR

[+Team LaLonde]

Thanks for all the input. I opted for Plan B. Frankly, choosing coords that I knew were not very accurate didn't seem like a responsible thing to do. Not that I'm a whiz or anything, but I trust my math and mapping skills more than I trust a GPSr with accuracy of nearly a hundred feet. I added a puzzle to the cache, making it our first 5/5.

Keep in mind that a true 5 on difficulty should be a serious challenge to find and require multiple trips to complete. A 5 on terrain is reserved for caches where special equipment is required to physically reach the cache location.

 

Most 5/5 caches are quite overrated on one scale or the other.

 

The cache can be accomplished by descending a cliff from the north or boating to the cliff from the south and climbing a short distance. Approaching from east or west is *nearly* impossible.

[+palmetto]

By the way, don't the reviewers use Google Maps to check the locations of new cache submittals?

 

Yes, BUT, if a distance is critical, I download an loc. and drop the coords onto the USGS aerials using Expert GPS. The USGS views themselves are out of date in my area, but the cache coordinates will be correctly located on the map. This prevents me from bothering someone about the RRtracks, which appear to be very near to their cache on Google Earth, when in fact the cache is sufficiently away. Or asking them about the private golf club, when their cache is in the roadway ROW island.

 

OT, I think work the coords in from an opening is correct as the OP has apparently done. The Topo zone map on the cache page should allow a check of the coords.

[GPS-Hermit]

If the cache co-ordinates cannot be obtained by using the GPS then don't post those co-ordinates at all. Give the finder a waypoint that is useable and just give distance and bearing from there. Use other information to narrow it down some more. Sounds like getting to it will be hard enough once you decide what your target is.

If the finder attempts to use the GPS to locate the cache at 90+ feet off of calculated co-ord's then he is at a big disadvantage. Rock cliffs will bounce a signal into uselessness.

[+Team LaLonde]

I had a first-finder on the cache in question. Apparently my method of calculating led him to the right spot.

[+WebChimp]

I had trouble while hiding a difficult cache in the side of a cliff today. The cliff was on the edge of a lake in a ..... it was snowing so hard I could only see about 150 feet...... accuracy from out on the ice was good.

 

Let me see if I have this right...........

 

You're putting a cache on the side of a cliff, above an iced-over lake, while it's snowing, with visibility down to 150'?

 

Man, that makes me glad I live in the South.

[crawil]

I had trouble while hiding a difficult cache in the side of a cliff today. The cliff was on the edge of a lake in a ..... it was snowing so hard I could only see about 150 feet...... accuracy from out on the ice was good.

 

Let me see if I have this right...........

 

You're putting a cache on the side of a cliff, above an iced-over lake, while it's snowing, with visibility down to 150'?

 

Man, that makes me glad I live in the South.