Tutorial: Introduction to GDAL

4. Convert GIS formats

4.4. Convert Comma Separated Values (CSV)

Sometimes you want to reproject coordinates in an ASCII file, e.g. which has been saved to text in a spreadsheet program. Here we will convert the coordinates in a comma separated ASCII file locations.csv to a new ASCII file locations_reprojected.csv.

It is good practice to first view the contents of a CSV file and to verify (1) the coordinates, and (2) the column separator. The separator is not always a comma and sometimes depends on the language settings used while exporting from a spreadsheet programme.

1. View the contents of locations.csv . You can use the type command from the command prompt as you learned in the Command Line tutorial.

We can see that coordinates are in lat/lon, which means that we can use EPSG:4326. The column separator is a comma. The last column gives objects with a string in quotes.

To change the projection of the CSV, we first have to create a virtual data source by creating an XML control file.

2. On your windows computer open notepad and type/copy the XML code given below. Use indentations of three spaces.

<OGRVRTDataSource>
   <OGRVRTLayer name="locations">
      <SrcDataSource>locations.csv</SrcDataSource>
      <GeometryType>wkbPoint</GeometryType>
      <LayerSRS>EPSG:4326</LayerSRS>
      <GeometryField encoding="PointFromColumns" x="lon" y="lat"/>
   </OGRVRTLayer>
</OGRVRTDataSource>

3. Save the file as locations.vrt in the gdal_exercises folder.

Some explanation about the XML file:

<OGRVRTLayer name="locations"> should correspond with the <SrcDataSource>locations.csv</SrcDataSource>
<LayerSRS>EPSG:4326</LayerSRS> should correspond with the EPSG code of the coordinate columns
<GeometryField encoding="PointFromColumns" x="lon" y="lat"/> indicates the columns with the coordinates that you want to convert.

4. Execute the following command:

ogr2ogr
-t_srs EPSG:28992 -f "CSV" locations_reprojected.csv locations.vrt -lco
GEOMETRY=AS_XY

In this example locations.csv with lat/lon WGS-84 coordinates is converted to locations_projected.csv with Amersfoort/RD New projection.

5. Use notepad to check locations_reprojected.csv. What is saved in each column?

In the same way we can convert the comma separated file to a shapefile.

6. Execute the following command:

ogr2ogr
-f "ESRI Shapefile" -t_srs
EPSG:28992 locations.shp locations.vrt

7. Check the result with ogrinfo.

8. Visualize the shapefile in QGIS by plotting the locations over the DEM, road map and Delft community border. Make a nice map.

9. Convert the locations file to Google KML, open in Google Earth and find out what the object locations are.