How do I guess which geographic projection system is used for a dataset?

Question

I have a bunch of shapefiles to convert to kml.

I can open them, but I don't understand the coordinate systems they are in (nor was it provided by the creators).

For instance, this should be a point on the border of the city of Amsterdam: 'x': 106980.0, 'y': 491810.0.

Ideally I'd love to find a website, a library, a script, an oracle, an octopus or anything that tells me what projection these data are in, knowing that the should roughly map to 52° N, 4.7° E.

Does anything like this exist? My curernt need is to find this specific projection system, but if possible I'd prefer a method to guess it for ll those cases where you have a couple of coordinates in an unknown format but you know where they should map to. Readers of this question would benefit from that too, and would not benefit from the answer to my really specific question.

Answer 1

Just try some informed guesses.

• Try the UTM zone. Be aware people sometimes use false origins.
• See whether the country has a national grid. Google is telling me about this ("Amersfoort", "Rijksdriehoeksnet", "RD"), EPSG:28992
• Any decent GIS will have a list of projected coordinate systems built in. Flick through the list looking for European/Dutch systems.
• Ask a local. For instance email a polite question to this map shop in Amsterdam or to ESRI Netherlands asking what coordinate systems are typically used there.
• Ask the person who created the data (I'm assuming there must be some reason why you can't do this)

Answer 2

For shapefiles you can run the opensource tool http://www.gdal.org/ogrinfo.html to see details on projection etc.

ogrinfo.exe  "C:\path\shapefile.shp" -al

alt text http://geographika.co.uk/images/ogr.png

You can download a whole suite of tools from http://fwtools.maptools.org/

This includes the http://www.gdal.org/ogr2ogr.html tool which you could use to convert from shapefile to KML as both are OGR (Open Geospatial Consortium vector) formats - http://www.gdal.org/ogr/ogr_formats.html

If you don't have a projection file or metadata then you can take a look at http://spatialreference.org/ref/epsg/28992/ which lists nearly all projections and provides projection parameters so you can add them into a GIS.

If you don't have a GIS you could set up http://www.proj4js.org/ to test a few points and convert into lat lon. It is probably best to get a GIS program though to visualise them - QGIS is an opensource option.

Answer 3

I really love this web tool: http://projfinder.com/ . Check your file and look up some coordinates. Zoom to approximately the same place on the globe and let it guess.

See also the other answers at GIS.stackexchange.com: https://gis.stackexchange.com/questions/7839/identifying-coordinate-system-of-shapefile-when-unknown