Map Datum Question

[DavidR]

The Carto Craft map of Sipsey Wilderness uses NAD27. I keep my GPS set on NAD27 CONUS. What is the difference between NAD27 CONUS & NAD27 CENTRAL?

[McDowra]

Good question and I don't know the answer, but leads to another question I have. Why did they put the coordinates in NAD27 when most applications are getting away from it and going WGS84.

here is a example of the differences

Normal GPS Coordinates (WGS84 Datum)
Decimal    34.08327 -87.2125
DDD MM.MMM    N 34° 04.996 W 087° 12.750
DDD MM SS.SSS    N 34° 4' 59.7720" W 87° 12' 45.0000"


Older Datum (NAD27)
DECIMAL    34.08314 -87.2125
DDD MM.MMM    N 34° 04.988 W 087° 12.750
DDD MM SS.SSS    N 34° 4' 59.2860" W 87° 12' 44.9928"


UTM (WGS84 Datum)
16S E 480395 N 3771409
British Grid
4156410 -8083107

[JC785]

I don't understand either, glad someone posted the question. Sorry for no help.

[DavidR]

Here's a great PDF from Garmin about using a GPS with a map.  http://www8.garmin.com/ma...hPaperLandMaps_Manual.pdf It goes into great detail about different types of maps and some different datum. I learned from this but still don't know the answer to my original question. The main reason I asked is that as McDowra illustrated, different datum state the same location in different ways. In order to accurately match a GPS to any given map, you have to know the right datum.

[ednotmilkman]

Is there no "map" with contour lines loaded into your GPS (Q)

All I do is look at where I am in relation to the contour lines and streams on my GPS map overlay to match the paper map.  I have no clue about all the different map datum stuff.  The only thing I have to watch is whether the GPS is aligned with True North or Magnetic north when I'm somewhere on earth where the declination is more than it is in Alabama.  If I'm using compass to get bearings on nearby peaks etc., I make sure the GPS is set to show Magnetic bearings. If I'm relying on a paper map, then keep the GPS oriented with True N and hope I remember to add the declination the right way when getting my bearings on the compass.

If I was hiking someplace like the Everglades, then I probably would not be able to find where I was on the paper map.

[Lostsheep]

I think the map confusion is one of the reasons, lots of people are pushing the UTM coordinates. Plus it's a whole lot simpler. Most topo maps put the datum in the bottom corner stating if it was NAD27 or WGS84. You'll just have to set the GPS to read the same. It'll do all your conversions for you.

The Datums are simply mathematical models of the Earth's shape and are not different map files. Each mathematical model is based on the projection of the map and who thinks it is most accurate. Each projection leads to different areas of higher inaccuracies.

NAD27 - North American Datum established 1927 (composed of several datums for North America ranging from Alaska to Central America - has more profound errors at the Northern and Southern ends of the spectrum)
NAD27 CONUS - North American Datum CONtinental United States established 1927 (mostly used for Navigation purposes specifically in the United States)
WGS84 - World Geodetic System established 1984 (most common datum type out as of now)

Hopefully this hasn't confused you too much! Oh and David - that's a great article from Garmin. Thanks for sharing.