Projection question

[+JohnCNA]

I have completed a couple of multi-caches using the projection method with my Garmin Etrex 30. Most of the projections have less than .2 miles or so.

 

I am now contemplating another multi that has a projection of around 4 miles away. Being someone who likes to prepare before I search, I checked the results that my Etrex 30 gives me vs CacheSense on a Blackberry vs online results at gpsvisualizer.com. Suprisingly, I received a variation of ~30 feet N/S and ~30 feet E/W or so between all three. I would like to believe gpsvisualizer is the most accurate since it shows the results to something like 6 digits after the decimal point. This variation concerns me a little.

 

I know where the setting in the Garmin is for magnetic vs true north and have it set to true north. I don't see this option in CacheSense or gpsvisualizer. Could this be part of the discrepancy? Is this just a difference in rounding in the different softwares?

 

I'm looking for some input from those who most likely would know more about this than I do.

[+dakboy]

Suprisingly, I received a variation of ~30 feet N/S and ~30 feet E/W or so between all three. I would like to believe gpsvisualizer is the most accurate since it shows the results to something like 6 digits after the decimal point.

Precision (number of decimal places) is not accuracy (correctness), nor does it give you any indication as to how accurate the calculation really is. If you're operating in the continental US, 3 decimal places (if you're using XX YY.YYY for your coord format) is plenty precise - changing that last digit by 1 means a difference of about 6 feet.

 

I know where the setting in the Garmin is for magnetic vs true north and have it set to true north. I don't see this option in CacheSense or gpsvisualizer. Could this be part of the discrepancy? Is this just a difference in rounding in the different softwares?

From your profile, it looks like you're in Illinois. Normally I'd say "30' over 4 miles is a smaller discrepancy than I'd expect for magnetic vs. true north" but your declination in IL is very close to 0 degrees, so it's quite possible that it's a true vs. magnetic difference. Have you tested the projection with the Garmin set to magnetic to see what you get?

 

Also, there are multiple ways to calculate the projection, using different models of the planet and different formulas (someone like Fizzy has forgotten far more about this than I'll ever know). If each piece of software (including what's in the Garmin) is written differently, you will get different results - regardless of rounding.

[+Mike & Jess]

Does the cache in question specify Magnetic vs. True North?

 

I own 2 caches that have projections, and I either give both, or specifically identify what I have given, and provide the estimated declination for the area to allow for adjustments (maybe I give the cacher too much info).

If no indication is given, fire off a quick note to the CO and ask.

 

Both mine have been shorter projections, with the longest being 241 meters away with no direct line of sight.

[+dprovan]

Assuming this isn't just magnetic vs. true confusion, take a look at FizzyCalc. It provides three different methods for calculating projections, so that might help sort out what's going on with the other tools. In my experience, caches involving significant projections always confirm the coordinates to within 20 or 30 feet instead of confirming them precisely, so I've always taking that kind of error to be a normal part of projecting.

 

By the way, for those shorter distances in the field, I find it easier to project in reverse: i.e., I just leave my GPSr set to the original coordinates and then walk to the point where my GPSr says the original base coordinates are the proper distance at a bearing of N+180.

[+CanadianRockies]

Be aware that even if you use very accurate software to do your projection, there's no guarantee that the cache owner did the same. If they used poor software, then the cache could be hidden a significant distance away from your projected GZ.

[+Mike & Jess]

For the cache in question, has there been may found logs, dnfs, notes, etc that would indicate major issues?

Is the cache fairly new or been around for a while?

 

It could be a fairly easy cache location to spot, and the margin of error may not be a huge concern.

[+JohnCNA]

For the cache in question, has there been may found logs, dnfs, notes, etc that would indicate major issues?

Is the cache fairly new or been around for a while?

 

It could be a fairly easy cache location to spot, and the margin of error may not be a huge concern.

For caches in the woods, 30 feet is pretty typical. Even urban hides get close to that some times, especially if they only take a single reading or use a phone. In the logs, some people calculated manually and were 100+ feet off then re-did their math and got closer. No real complaints from those with GPS's that could do it for them. Most were within reasonalbe distances.

 

I just thought it was odd that 3 different dedicated solutions gave 3 different answers.