The short explanation
All spatial data contains coordinates that specify where features should be drawn on the earth’s surface. For historical and technical reasons, there are numerous coordinate systems used to define these coordinates.
In order to overlay your data with basemap data in Mango, we automatically transform your data to a common coordinate system when you upload it.
In most cases, the transformation will work fine, but in a minority of cases the transformation from your coordinate system to our standard may not be supported. In these cases, you may not receive an error message, but the error may manifest itself as an offset between your data and the base data.
In these rare cases, you will need to convert your data to a standard coordinate system (e.g. WGS84\Lat Long) using a desktop GIS.
The longer explanation
When you upload data to Mango, a projection file (.prj) in Well Known Text (WKT) format is required. When reading the data, Mango will automatically detects the coordinate system of your data and converts it to the coordinate system used by Mango.
We use the Spherical Mercator Coordinate (EPSG:3857) system to allow integration with other web map services such as Open Street Maps.
Note: Spherical Mercator’s EPSG code is 3857, but the unofficial code 900913 (google, get it?) is used widely by Google and others.
During the upload process, Mango will automatically perform a coordinate system transformation, based on OSGEO’s PROJ.4 library. Our ability to perform a datum transformation will depend on whether the PROJ.4 library has a datum transformation defined, in particular, the TOWGS84 parameter (as in the example below) or NADGRIDS parameter.
PROJCS["GGRS87 / Greek Grid",
You can find out if your datum is supported by searching spatialreference.org and looking for the TOWGS84 and NADGRIDS entries.
If it is not found, you will need to convert your data to a WGS84 based coordinate system using desktop GIS software before uploading to Mango.