Tuesday, February 07, 2006

Know Your NADs

Are those GPS coordinates NAD27? NAD83? WGS84?? What I'm talking about is the "datum", or ellipse model used to approximate the shape of the earth - it's not a sphere because it bulges around the equator. Coordinates for the same location, but using a different datum, will have different values.

Way back in 1866, somebody started in Kansas and manually surveyed across the continent. The government used that data back in 1927 to come up with a model of the earth's shape, NAD27 for North American Datum 1927, and that's what the USGS used for all their maps. Believe it or not, manually surveying the planet back in 1866 turned out to not be exactly correct, so in the 80's they used all that Star Wars technology to make a new model of the earth, NAD83. This model was expanded to work for the entire planet instead of just our continent, that's called WGS84.

I use WGS84 because that's what Google Maps uses, and it's probably what most GPS devices default to. However, all the old USGS maps use NAD27. My GPS stores waypoints in some agnostic format so all I have to do is switch the datum it's using and all my points are magically converted for me.

As chance would have it, WV's location happens to fall so that both NAD27 and NAD83/WGS84 are pretty close together - within tens of meters. They're almost the same near TAG, but 300 meters off towards the West coast, as seen below.

NAD27 NAD83 Difference

The difference between NAD27 and NAD83 coordinates by location

No comments: