Benchmark Map Datum vs Geocache Datum

[Readymixer]

Forgive me if this is covered in the FAQ but I could not find a resolution there. I have noticed that benchmark coordinates are in NAD83 and cache coordinates are in WGS84. Will this cause me not to be able to find a mark if my unit is set to WGS84? Also, will I do better, in general, if I use the descriptions tahn the GPSr since there was no GPS when most of these markers were set?

 

If you shoot for the moon and only get half way, think about how far you've gone.

[+embra]

NAD83 and WGS84 are virtually identical, so that won't be a problem. See this discussion for some opinions on how helpful a GPSr is in searching for benchmarks, though. Your speculation that no GPS is necessarily needed is right.

 

Don't get even - get odd

[+GEO*Trailblazer 1]

From its time of inception the measurement of longitude and lattitude have been the form of measurement that geodetical surveys were based on,the Greenwich Meridian and the Equator.The changes of the Meridian hour are,Before the Greenwhich hour was accepted the local surveyor relied on his own true meridian hour at noon.Acceptence of Greenwhich till 1986,then till now time is based on the atomic clock,IE,UNIVERSAL TIME,so you can see that is why you have to know the declination of the years of the surveys.The Magnetic Field has moved approximately 7.5 degrees since 1846 see isogonic charts,to the WEST,called annual westward slippage,the North Star has also moved,Proper motion of the Stars, but I do not have the data base nor the required instrument to show this,only by evidence from the Pyramids,that show that if you go back in time the North Star Lines up with the Kings Chamber.And that is how they were so astronomicaly,GEODETICALY accurate they were then.After all that is where all the knowledge came from all the ANCIENTS ONES.......................

 

When all else fails Geotry again.

[+Kewaneh & Shark]

Embra is right - NAD83 and WGS84 are virtually identical. Most horizontal datums are, and the recreational GPSr's we use aren't accurate enough to tell the difference between most of them. What needs to be remembered when searching for the benchmarks is that the coordinates shown on data sheet are some times (many times) calculated coordinates and not actually derived by occupying the mark with a theodolite or sextant, let alone a GPSr.

 

I've found benchmarks that my Magellan has taken me to within a few feet of the posted coordinate, and I've found others that were off by as much as 350-400 feet. One of my recent finds was off 0.21 miles! What I've found is that most of the older benchmarks that have not been occupied in the last 10-15 years (pre-GPS), have the more erratic posted coordinates. The most accurate coodinates I've found have been on the marks recently visited by the DOT during a state-wide HPGN (High-Precision Geodetic Network) densification survey. My Magellan literally zeroed out on one of them. THAT was cool.

 

Simply put: Use the GPSr to get you close, then rely on the descrption to help you actually find it.

 

Keep on Caching!

- Kewaneh