GPSr Coordinate Conversion Question

[+WolfBlitzer]

I'm relatively new to the 'sport' of Geocaching. We have about 25 finds under our belt, and after today 2 DNFs

 

I'm well aware that I'm using the 'wrong' type of GPSr for caching (Magellan Maestro 3225) but it has served us well thus far, coupled with printed maps from maps.google.com and taking clues with us on the Palm Pilot.

 

This GPSr is great in the car and has a somewhat useful digital compass on the same page that shows your Lat/Lon in Degrees, Minutes & Seconds.

 

My question is, most of the caches on geocaching.com list their coordinates using Degrees and Minutes (MinDec) and when I'm using the POI Editor software to enter in a cache to load onto the GPSr it wants me to use the following format (according to the Help file):

 

Latitude (in degrees and fraction of a degree; example: 45.254°North).

Latitudes range from 0° to 90° and are either "North" or "South".

 

Select "North" if the POI is located in the northern hemisphere (i.e. between the Equator and the North Pole)

Or select "South" if it's located in the southern hemisphere (i.e. between the Equator and the South Pole)

 

Longitude (in degrees and fraction of a degree; example: 122.542°West).

Longitudes range from 0° to 180° and are either "West" or "East".

 

Select "West" if the POI is located west of the prime meridian (Americas, western parts of Europe and Africa, etc.).

Or select "East" if the POI is located east of the prime meridian (Asia, eastern parts of Europe and Africa, etc.)

(The prime meridian goes through Greenwich near London (UK) and gives the 0° reference for longitudes).

 

I just want to know the proper coordinate type to give the POI Editor, and also the best way to use the Lat/Lon coordinates that the GPSr gives me on the screen to make sure I'm accurately in the area... if that makes any sense.

Edited July 15, 2008 by WolfBlitzer

[+WolfBlitzer]

I guess what I'm also asking is, is the POI Editor looking for Decimal Degrees (WGS84) data or some other type? I couldn't find anywhere in the manual that said that this particular unit uses WGS84 or NADxx so I'm just assuming it's using WGS84.

[+coggins]

To the right of the co-ords on each cache page is a link that says "other conversions...", you can use that to get the Decimal degrees you need for your POI loader, and the DDD MM SS.SSS for the compass. Give it a look and see if this helps. For multis and unknown caches, you might look into downloading FizzyCalc.

Edited July 15, 2008 by coggins

[+WolfBlitzer]

To clarify, using an online coordinate conversion, or "other coordinates" link from the cache's page, it may show GPS coordinates as:

 

N 45° 32' 10.7412"

 

But when I'm using this GPSr it would only show me at:

 

N 45° 32' 10"

 

making it hard for me to know exactly where to be looking.

 

As I said, I know this isn't the best GPSr to be using for this, and I could probably get a cheaper handheld unit to use that will give me more detailed seconds (or even able to choose what coordinates I want to use)

 

But I'm just worried I'm overlooking something obvious here with my entries, conversions and displays and would just appreciate a second or third set of more experienced eyeballs to help out a couple of noobs.

Edited July 15, 2008 by WolfBlitzer

[andylphoto]

Let me first say that I am completely unfamiliar with that particular GPS.

 

That said, you may have two different issues going on. From your first post, the example of

 

45.254°North

122.542°West

 

would tell me that what your GPS wants to see for the POI is what is referred to as decimal degrees, which can be derived in either way mentioned above--on the cache page, or through an online conversion.

 

In your last post, the other problem may be the accuracy of the GPS. The numbers you post there appear as Degrees, Minutes and Seconds, or DMS. However, it looks like the GPS will only display your position to the nearest second. That type of accuracy should get you within 50 feet or so, plus the expected position error of the GPS.

 

One second of latitude is about 100 feet, so when your GPS gives your position, the position of the cache could be 50 feet either way, due to the rounding of the coordinates. Longitude is about the same at the equator, but the distance gets smaller as you approach either pole. As I mentioned, the expected position error of the GPS could add to that error, or reduce it. You also may find certain instances where it will be right on the money--but there's the problem. There's lots of room for error. Even if you figure only 40 feet of longitude and 50 feet of latitude in each direction for potential rounding errors, that's potentially 8000 square feet of search area. If the positional error of your GPS happens to be in the same direction as the rounding error, you may have just expanded your search area.

 

If you're serious about geocaching, you might want to consider picking up an inexpensive handheld. There are entry level units that retail new for around $100. Most GPS units that I'm aware of will only display DD MM SS.S but adding that one tenth of a second drops your rounding error down to a distance of 10 feet--five in either direction, which is going to be less error than the potential position error of the GPS in most cases.

[+twolpert]

I also noticed that you mentioned the WGS84 datum as if it was somehow related to the format (and precision) of the coordinates. This is not the case. The datum is an entirely different issue. A datum is a way of relating coordinates to a particular point on a map. Different datums will have slightly different coordinates for the same point. Cache coordinates are always stated in the WGS84 datum, so you should always keep your unit set to that datum.

[jimlarkey]

Just a followup question on the datum discussion........does the GPSr (Garmin) datum setting have to match the mapping software datum (Mapsource, MapTech, ExpertGPS, etc) when transferring waypoints?

 

Stated another way, does the software in the GPSr and mapping software coordinate differences in datum settings?

 

Thanks, and Cheers,

Jim

[+trainlove]

Just a followup question on the datum discussion........does the GPSr (Garmin) datum setting have to match the mapping software datum (Mapsource, MapTech, ExpertGPS, etc) when transferring waypoints?

 

Stated another way, does the software in the GPSr and mapping software coordinate differences in datum settings?

 

Thanks, and Cheers,

Jim

 

Yes, the datum must match your mapping software.

It's almost not important when using electronic maps as they mostly use WGS84.

It really only comes into play when you try to pencil plot a waypoint on a paper map. The difference can be a hundred meters or so and that is a big deal on paper.

 

One second of latitude is about 100 feet, so when your GPS gives your position, the position of the cache could be 50 feet either way, due to the rounding of the coordinates.

 

Well really you could still be 100 feet off, not 50 feet. And if you are perfectly in the wrong spot and the cache is in the wrong spot you really could be 200 feet off.

Edited July 16, 2008 by trainlove

[+WolfBlitzer]

Thanks guys for your help.

 

Unfortunately this automotive-type GPSr that we've been using does not allow me to switch datum types on it (nor switch from magnetic/true North) so I will just have to learn to live with it's inadequacies until I either get better or get enough money for a hand held that will give me coordinates with more detailed seconds.

 

You fellow cachers have been extremely friendly, patient and helpful in helping Amy and I get up to speed.

 

My next step is to learn about map/compass orienteering. Not solely for geocaching purposes, but to also have that necessary survival skill under my belt "just in case"

[jimlarkey]

Just a followup on the previous poster's thoughts.......with Garmin products you DO NOT have to match the datum of the software and GPSr. Rec'd the following from tech support (and it does make sense):

 

Thank you for contacting Garmin International.

 

When you transfer waypoints from the 60CSx into Mapsource, Mapsource

will convert the waypoints to datum Mapsource is using. Most other

programs will either automatically covert the points to the other datum

or give you an error message asking you to change the datum before the

answer.

 

 

 

Yes, the datum must match your mapping software.

It's almost not important when using electronic maps as they mostly use WGS84.

It really only comes into play when you try to pencil plot a waypoint on a paper map. The difference can be a hundred meters or so and that is a big deal on paper.

 

One second of latitude is about 100 feet, so when your GPS gives your position, the position of the cache could be 50 feet either way, due to the rounding of the coordinates.

 

Well really you could still be 100 feet off, not 50 feet. And if you are perfectly in the wrong spot and the cache is in the wrong spot you really could be 200 feet off.