GPS accuracy

[Icrewedhuey]

I went out on my first geocaching this morning and although I located the caches my GPS had me WSW on both what it indicated as .002 to .004 mi. It looked to be about 100 yards or so off. I have a Magellan 300. Is there any way to get more accurate with this unit? I would like to get more involved in this but I think I will need better accuracy than that. Any help for a new guy?

[BassoonPilot]

quote:

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Originally posted by Icrewedhuey:

I went out on my first geocaching this morning and although I located the caches my GPS had me WSW on both what it indicated as .002 to .004 mi. It looked to be about 100 yards or so off. I have a Magellan 300. Is there any way to get more accurate with this unit? I would like to get more involved in this but I think I will need better accuracy than that. Any help for a new guy?

---

 

Well, I used a Magellan 300 for my first 170-or-so finds, and my daughter continues to use it when she goes geocaching with me. You should realize it is an older model; a 1999 vintage, and the software can't be upgraded. Unless things have changed since I purchased mine, the Maggie 300 only reads to two decimal places, or hundredths of a mile. That means with a gps indication of 0.00 you could be as far as 50 feet from the posted, three decimal place, coordinates. (Assuming the coordinates were accurate in the first place.) When one is standing still or moving slower than 2.5 miles an hour, one must remember to wave the Maggie 300 in a wide arc frequently (you'll see it referred to in the forums as the "hula wave") to break averaging.

 

I found that for most of the caches I found with my Maggie 300, the gps indicated 0.01 or 0.02 miles from the cache; really that's close enough if you are pretty observant.

 

Some users opt to set the Maggie 300 to read in km/kph instead of mi/mph on the theory that 1/100 of a kilometer is smaller than 1/100th of a mile. I tried this method too, but didn't notice any increase in accuracy.

 

I had a lot of fun using my Maggie 300, and I hope you do, as well.

[Kerry.]

100 yards off (and WSW) might indicate the incorrect datum, should be WGS84 or NAD83 not any of the NAD27's etc.

 

The 0.02 to 0.04 mi (I assume you mean miles and not minutes?) is still short of 100 yards but then so is the precision capability of some GPSR's.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

100 yards off (and WSW) might indicate the incorrect datum, should be WGS84 or NAD83 not any of the NAD27's etc.

 

The 0.02 to 0.04 mi (I assume you mean miles and not minutes?) is still short of 100 yards but then so is the precision capability of some GPSR's.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+The Commissar!]

I see posts indicating that coordinates are wrong because "My GPS" says different...My opinion is that each and every unit regardless of brand and model will at times vary and you can "learn" the habits of your individual unit...I use an even older Magellan Trailblazer XL (1996) and rarely cannot find what I'm looking for...I have a Meridian Plat. on order and will keep the XL for backup because it usually gets me to the point within 0.01 miles (about 56 feet) or closer at which time it is best (again in my opinion) to stop looking at the GPS and start looking for the cache or whatever...

[+The Commissar!]

I see posts indicating that coordinates are wrong because "My GPS" says different...My opinion is that each and every unit regardless of brand and model will at times vary and you can "learn" the habits of your individual unit...I use an even older Magellan Trailblazer XL (1996) and rarely cannot find what I'm looking for...I have a Meridian Plat. on order and will keep the XL for backup because it usually gets me to the point within 0.01 miles (about 56 feet) or closer at which time it is best (again in my opinion) to stop looking at the GPS and start looking for the cache or whatever...

[+Furby & the Bead Babe]

If you are not using a map to find location, is map datum still important? I read here that you should not use nad 27. Most of our maps use nad 27 in my area. I use nad 27 conus. Why would you use another map datum? I live in Washington State on the west side. I realize if you use a compass to set your unit to mag-north and true north if not using a compass. Any thoughts?

[Atilla the Pun]

The default datum for Geocaching.com is WGS-84. If you have your GPS set to a different datum then all of your co-ords will be off, even if you are using the same grid (UTM or Lat/Lon, etc.) listed on the cache page.

 

AtP

[JasonW]

quote:

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Originally posted by Jim M. Furubotten:

If you are not using a map to find location, is map datum still important?

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The whole thing stems from the fact that the Earth is not a perfectly definable simple spheroidal shape so each of the map datums is making a representation of the earth - these representations are a pretty good fit, but not perfect and they vary in different ways.

 

Imagine you want to describe where the USPS depot is in your town - if you describe it with respect to the Police Department you have one representation, if you do it with respect to the Fire Department you have another representation - both equally valid, but if you're just given the direction and have no clue whether it's based on starting at the PD or the FD you won't end up in the same place.

 

Now read that paragraph again but substitute the following words: USPS Depot = Geocache, Police Department = NAD27 datum, Fire Department = WGS84 datum.

 

Does it make any more sense ?

 

----8<--------8<---CUT-HERE---8<--------8<----

[Kerry.]

quote:

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Originally posted by Jim M. Furubotten:

If you are not using a map to find location, is map datum still important? I read here that you should not use nad 27. Most of our maps use nad 27 in my area. I use nad 27 conus. Why would you use another map datum?

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Very important as it's a lot more than just a map datum (as such) as ALL coordinates to be of any use require a datum map or no map. This equally applies to UTM in which case the zone is also required.

 

One needs to be using the datum that appplies to a particular data/coordinate set. If one is working from/on a NAD27 map then it's NAD27, a Geocaching coordinate set then WGS84 which also assumes the person who placed them was (well should have been) also working in WGS84. If not then the ground positions based on the same coordinate set used with a different datum will not match.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

quote:

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Originally posted by Jim M. Furubotten:

If you are not using a map to find location, is map datum still important? I read here that you should not use nad 27. Most of our maps use nad 27 in my area. I use nad 27 conus. Why would you use another map datum?

---

 

Very important as it's a lot more than just a map datum (as such) as ALL coordinates to be of any use require a datum map or no map. This equally applies to UTM in which case the zone is also required.

 

One needs to be using the datum that appplies to a particular data/coordinate set. If one is working from/on a NAD27 map then it's NAD27, a Geocaching coordinate set then WGS84 which also assumes the person who placed them was (well should have been) also working in WGS84. If not then the ground positions based on the same coordinate set used with a different datum will not match.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Alan2]

quote:

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Originally posted by Jim M. Furubotten:

If you are not using a map to find location, is map datum still important? I read here that you should not use nad 27. Most of our maps use nad 27 in my area. I use nad 27 conus. Why would you use another map datum? I live in Washington State on the west side. I realize if you use a compass to set your unit to mag-north and true north if not using a compass. Any thoughts?

---

 

If some one gives you coordinates in WGS84 Datum, then you set WGS84 in your GPS before entering the coordinates. THis happens most of the time since the coordinates on this web site is listed in WGS84 Datum.

 

However if someone gives you coordinates in NAD27, then you set you GPS Datum to NAD27 and then enter the coordinates. This might happen if someone is reading from one of your west coast maps for example. When you report the coordinates to this site, then you switch the GPS to WGS84 and then report the coordinates.

 

Conversely, if you want to see where you are on a paper map that's in NAD 27, then change the Datum to NAD 27 in your GPS and the coordinates will match the paper map.

 

Phew.

 

Alan

[+Alan2]

quote:

---

Originally posted by Jim M. Furubotten:

If you are not using a map to find location, is map datum still important? I read here that you should not use nad 27. Most of our maps use nad 27 in my area. I use nad 27 conus. Why would you use another map datum? I live in Washington State on the west side. I realize if you use a compass to set your unit to mag-north and true north if not using a compass. Any thoughts?

---

 

If some one gives you coordinates in WGS84 Datum, then you set WGS84 in your GPS before entering the coordinates. THis happens most of the time since the coordinates on this web site is listed in WGS84 Datum.

 

However if someone gives you coordinates in NAD27, then you set you GPS Datum to NAD27 and then enter the coordinates. This might happen if someone is reading from one of your west coast maps for example. When you report the coordinates to this site, then you switch the GPS to WGS84 and then report the coordinates.

 

Conversely, if you want to see where you are on a paper map that's in NAD 27, then change the Datum to NAD 27 in your GPS and the coordinates will match the paper map.

 

Phew.

 

Alan