The geographic reference system of a shapefile (RS) or another kind of vector in a GIS environment is a fundamental element to allow the correct projection of our geographic objects (points, lines, areas) on a map. However, switching from one system to another is not automatic, and sometimes you could get an incorrect positioning of your features.
(the property menu of a shapefile with the coordinate system in Qgis)
If using a GIS application such as ArcGIS or QGIS we take a shapefile that has a coordinate system, for example Monte Mario Italy 1 (one of the most used by public administrations in Italy) and we simply try to change the RS (again for example) in WGS_1984_UTM_Zone_32N – EPSG 4326 (another popular RS used in Italy), we will see the shapefile shifted, with an error that can be relevant, resulting in an incorrect positioning in the space of the features contained in the file.
In practice it is necessary to perform a specific transformation action to obtain a file correctly positioned in the new RS.
So, working in ArcGIS (Esri ArcMap) you have to:
- create a new data frame with coordinates WGS_1984_UTM_Zone_32N
- load the file in Monte Mario Italy 1 coordinates and approve the transformation “Monte_Mario_To_WGS_1984_2” for the reprojection in WGS84 (Arcgis allows the reprojection “on the fly” between different coordinate systems). It is suggested to use the transformation 2 (Monte_Mario_To_WGS_1984_2), very accurate for the modification of the coordinates for Italy.
- right click – Data – Export data selecting as Coordinate system “the data frame” in order to export in WGS84 (coordinate system of the data frame)
- save the file with an appropriate name and different from the starting file. I often add the suffix wgs84 to be sure of the shapefile coordinates
- it is always good to do a final check of the coordinates, to be sure of the actual conversion, by opening the file in ArcGIS or QGIS and examining the shapefile properties.
It is obvious to say that the transition towards other coordinate systems has to be performed in a similar way .
At this point you can open the file directly in ArcGIS or in QGIS in a project file with the proper reference system WGS_1984_UTM_Zone_32N – EPSG 4326. The projections will coincide, and by loading for example in WMS the orthophotos of Google or Bing (from the Openlayers plugin) it will be possible to precisely position the boundaries of the shapefiles.
Using QGIS, the reference system change can be done with the Reproject layer command, which can be accessed from the menu Vector -> Data Management Tools -> Reproject layer, as shown in the following picture. In this case you have to select the file to transform, indicate the final CRS and specify the name of the reprojected file.
Note: ArcGIS is a registered trademark and one of the components of the ESRI platforms.
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