Projections issues

Some users have reported discrepancies with the location of their data over the basemap imagery in iGIS and/or a desktop GIS tool (such as ArcMap). If you are noticing a shift in your layers, this article may help you.

All vector data in iGIS is stored in the common coordinate system: WGS84 EPSG:4326. All raster data in iGIS (pre-processed through MapTiler software) is stored in the common coordinate system: EPSG:900913. Using these coordinate systems for all source data speeds up rendering in iGIS’s map display.

iGIS uses the open source proj4 library ( http://trac.osgeo.org/proj/ ) to support:

  • the reprojection of vector data on import/export from its original defined input coordinate system to WGS 84 (EPSG:4326)
  • re-projection of displayed iGIS project map centre coordinates from the WGS84 coordinate system into the projection defined for a particular project.

If you are unsure which coordinate system to use when you create a new project, we recommend you use the default EPSG:4326 WGS84 Lat/Lon for vector data.  This will ensure your data collection is not affected by reprojection within the application.

If you find that your points don’t line up with your basemap imagery as expected, it is likely that there is a bug with the proj4 library.  Bugs in the library can be submitted at the Proj4 website http://trac.osgeo.org/proj/ We incrementally update iGIS’s version of the Proj4 library with the latest available version, so when a fix becomes available in the library it will make its way into iGIS. Until then, the only workaround is to transform your data into a supported coordinate system, i.e. EPSG 4326 – WGS 84, with third party software like ArcGIS, prior to iGIS import.