7 replies to this topic

[benbakelaar]

Hello all, it's been a while... hope to be active again over this summer, I have a few projects lined up involving GIS and Gmaps...

So this is probably the simplest thing, but somehow I am missing it. I downloaded the NJ county shapefiles from http://www.census.go...cob/co2000.html.

I open it in MapWindow GIS. All I want to do is re-project it to WGS1984 so that I can overlay on a Google Map (using Shape2Earth, a MapWindow plugin). But the counties come unprojected. See metadata reference here: http://www.census.go...o_metadata.html. It does mention NAD1983 but if you delve deeper you will find statements clearly saying it is "unprojected".

I have tried re-projecting the files from various starting points, but none overlay properly in Google Maps, and for that matter, with any standard projected shapefile. The closest I've gotten is a tiny speck somewhere in Maryland... if I zoom to layer on the Census county shapefile, NJ shows up. If I zoom to the projected layer, it's not even 1 pixel big.

Any ideas?

[benbakelaar]

Sheesh, I swear I spent an hour trying different combos yesterday. I guess I picked the right one today, it *was* in "North American Datum 1983" originally. So I got it.

Not sure why Census clearly states it is unprojected. Maybe I don't understand the difference between a datum and a projection?

[Jimbo]

"All I want to do is re-project it to WGS1984"
The problem with Google Maps API java code you are limited to about two or three projections, I think WGS '84 is not one of them that you can use.

[pfyfield]

"Unprojected" means that the data are in geographic coordinates (lat-long).

[benbakelaar]

"Unprojected" means that the data are in geographic coordinates (lat-long).

Pat, in that case, shouldn't I have been able to re-project the shapefile into WGS1984 without pre-selecting the "existing" projection? I tried re-projecting by choosing various initial projections, including "Custom Projection"... there was no "Unprojected" option... but after re-projecting to WGS1984 and then overlaying on Google Maps, the only one that worked was when I selected "North American Datum 1983" as the pre-existing projection of the shapefile.

[Hans van der Maarel]

"Unprojected" means that the data are in geographic coordinates (lat-long).

Pat, in that case, shouldn't I have been able to re-project the shapefile into WGS1984 without pre-selecting the "existing" projection? I tried re-projecting by choosing various initial projections, including "Custom Projection"... there was no "Unprojected" option... but after re-projecting to WGS1984 and then overlaying on Google Maps, the only one that worked was when I selected "North American Datum 1983" as the pre-existing projection of the shapefile.

"Unprojected" still doesn't tell you what datum the coordinates are for. Unprojected WGS84 lat/lon isn't the same as unprojected NAD83 lat/lon and if you move from one datum to the other, you need to know what you're starting out with.

[Rob]

Maybe I don't understand the difference between a datum and a projection?

This might help clarify for you:

http://www.connect.net/jbanta/FAQ.html

[benbakelaar]

Maybe I don't understand the difference between a datum and a projection?

This might help clarify for you:

http://www.connect.net/jbanta/FAQ.html

Thanks Rob, that was really helpful and informative!