Data layers often need to be “projected” to get them into the same coordinate system as other layers. For example, if data comes from different places (e.g. online, governmental agencies) or represents different areas on the map, there is a good chance that the coordinate systems between layers will differ, therefore a projection of the data is required. If layers are not in the same coordinate system and not overlapping correctly, there will be errors in the resulting layers created after analyses. The Project (Data Management) tool is used to change a vector layer from one coordinate system to another. The same process can also be used for raster layers, but the Project Raster (Data Management) tool is used instead. If the layer’s coordinate system is unknown, it must first be defined, either in the layer properties in ArcCatalog or by using the Define Projection (Data Management) tool in ArcMap. In this example, we will project the colony_communes layer from the Swiss projection to WGS 1984 and import the results into Google Earth (KML file) (6.5.9.).
To use the project tool (Fig. 34):
- Type “project” in the Search window (or browse to Data Management Tools > Projections and Transformations > Feature > Project in the ArcToolbox).
- Select the Project (Data Management) tool.
- Input layer: colony_communes.
- The coordinate system of the layer will automatically be added.
- Save the output as: commune_wgs84.shp.
Indicating the new coordinate system in the name is useful for layer organisation.
- Choose the output coordinate system.
- Click the Browse button.
- Expand the Geographic coordinate systems option.
- Click World.
- Select the WGS 1984 coordinate system.
- Select a transformation from the predefined list. In this case, the first is sufficient.
- Click OK.
- Click OK again.
The resulting layer will possess the new projection and is
automatically added to the map document (Fig. 35). The green commune_wgs84
polygon layer is slightly shifted to the south compared to the blue colony_communes
layer underneath. As another example of the differences in coordinate systems,
Fig. 36 shows an example from Switzerland
between the WGS 1984 coordinate system (a) and the CH1903 projection (b).
Fig. 34. Dialog box of the Project tool.
Fig. 35. Results of projection showing the disparities in the two polygon data layers between the CH1903 and the WGS84 projections. Reproduced with permission of Swisstopo (BA13016).
Fig. 36. World Geodetic System (WGS 84, a) versus the Swiss projection (CH1903 LV03, b). Reproduced with permission of Swisstopo (BA13016).