Oaxaca Man

[Oaxaca Man]

I am brand new to geocaching, but I've owned a Magellan Sport Trak

unit for a couple of years and use it mainly when fishing in Canada and

have been very satisfied with it.

 

But, now that I want to use it for geocaching, I find it does not lead me

to the cache coordinates posted. I thought at first it was a battery

problem, so I put in new batteries. It didn't help.

 

Yesterday I tried to locate a cache - but knew for certain I was way off (like a couple of blocks). I also tested it against a couple of bench marks and was still way off.

For example, one BM was N 42 01.950 / W 091 35.933 and my gps read N 42 02.107

/ W 091 35.882. (But I'm not sure BMs are all that accurate.)

 

Today I tried again with a different unit, which did lead us to the cache area (although we were unable to find because of heavy snow). I know we were very close because of discription.

 

Can anyone tell me what I might be doing wrong? Or could it be my

Magellan? I have tried (unsuccessfully) to contact Magellan for their

imput.

 

I'd appreciate any suggestions.

 

Oaxaca Man

[+Sputnik 57]

My bet is that you need to change the "datum" in your SportTrack to WSG 84. If it is set to NAD 27, you're likely to be a couple of hundred meters off. Also, although it sounds like you have it right, be sure that your coordinates are set to hdd° mm.mmm (hemisphere degrees, minutes.decimal minutes).

 

Benchmarks that are listed as "scaled" are not too accurate (someone guessed their coordinates from a map). If the benchmark is "adjusted" its coordinates should be dead on.

Edited December 5, 2005 by Sputnik 57

[+I.R. Geonut]

Wow, good info. What's the difference between primary and secondary map datum?

Edited December 6, 2005 by IRBaboon

[Photobuff]

As said above, if you're getting your benchmark coordinates from a data sheet, and they're scaled, not adjusted, they can be *way* off. Around here, hundreds of feet is common. Combine that with the error of the GPS, and it can look pretty bad. I have a benchmark with adjusted coordinates up the road, and my 210 is typically within 9 feet if the signal is good. Adjusted coordinates will be good to some very small value, like under an inch. Don't expect your elevation number to be terribly accurate under the best of conditions.

[+Sputnik 57]

Wow, good info. What's the difference between primary and secondary map datum?

I'm not sure. Where are you seeing a setting for "secondary" map datum?

[stevesisti]

My bet is that you need to change the "datum" in your SportTrack to WSG 84. If it is set to NAD 27, you're likely to be a couple of hundred meters off. Also, although it sounds like you have it right, be sure that your coordinates are set to hdd° mm.mmm (hemisphere degrees, minutes.decimal minutes).

 

Benchmarks that are listed as "scaled" are not too accurate (someone guessed their coordinates from a map). If the benchmark is "adjusted" its coordinates should be dead on.

Hmmm, a couple hunderd meters off because of the NAD27 vs WGS84 seems like way too much. Have a look here a chart Peter Dana produced to get a rough idea of what the difference would be.

 

http://www.colorado.edu/geography/gcraft/n...m/gif/shift.gif

[+Sputnik 57]

You're right. A few hunded feet may be more accurate. From what I understand, though, the offset varies depending upon where you are on the globe. Judging by Oaxaca Man's benchmark, I'd put him somewhere near Cedar Rapids, Iowa.

[moishe]

this makes for a good question... How do you find a benchmark? I tried to find two caches not too long ago that were close to each other. I couldn't find either but I could tell I was at least in the ballpark...

 

I know my datum is set to wsg84. A benchmark might help.

[+Cornix]

What's the difference between primary and secondary map datum?

On the position screen of your SporTrak you can display the coordinates in two different formats (primary & secondary) at the same time.

 

I would select the following:

 

Primary coordinate system: Lat/Lon (DEG/MIN.MMM)

Primary map datum: WGS84

 

Secondary coordinate system: UTM

Secondary map datum: WGS84

 

Cornix

[+Sputnik 57]

this makes for a good question... How do you find a benchmark? I tried to find two caches not too long ago that were close to each other. I couldn't find either but I could tell I was at least in the ballpark...

 

I know my datum is set to wsg84. A benchmark might help.

From the main page on the geocaching web site, click on "find a benchmark" on the far right (or click here).

[Oaxaca Man]

Rgarding the options on the Sport Track Map Datum, I have two different WG options:

 

Primary: WG 572

Secondary: WG 584

 

I selected them and it seems to work fine. But one commentor suggested that there should be WG 584 options for both Primary and Secondary. Mine doesn't ... but should it make that big a difference if I use the options listed above?

[+welch]

Rgarding the options on the Sport Track Map Datum, I have two different WG options:

 

Primary:  WG 572

Secondary: WG 584

 

I selected them and it seems to work fine.  But one commentor suggested that there should be WG 584 options for both Primary and Secondary.  Mine doesn't ... but should it make that big a difference if I use the options listed above?

its WGS (world geodestic survey) not WG5

 

I think you should change your primary to WGS-84. I don't have any magellans to so I may be wrong, but I think the secondary is just to so you can display your location in a second setting. But I'm wondering if you input something (or use 'goto') if the unit is using the datum set as 'primary'? To me it would make sense it would.

Edited December 7, 2005 by welch

[Oaxaca Man]

Whoops. I guess that is WGS72 and WGS84. Sorry, I thought the "S" was a "5". But my point is the same.javascript:emoticon('')

[Oaxaca Man]

Can I whooops again? I stand corrected. There was a WGS84 option in my primary so now they are both (primary and secondary) set there. Thanks for the imput.

[4x4van]

On my Sportrak Pro, I set the Primary to WGS-84, DD MM.MMM, but I set the Secondary to NAD-27, UTM. That way, my secondary coordinates match paper topo maps (they are usually NAD-27).