Tag Archives: kml

More radio amateur grid squares

Toronto, as understood by the Maidenhead Locator system

After yesterday’s post, I went a bit nuts with working out the whole amateur radio grid locator thing (not that I’m currently likely to use it, though). I’d hoped to provide a shapefile of the entire world, but that would be too big for the format’s 2GB file size limit.

What I can give you, though, is:

  • A Perl program that will generate a shapefile of an entire Maidenhead grid field, down to the subsquare level: make_grid.pl. You’ll need Geo::Shapelib to make this work. 324 (= 182) of these files would cover the whole world, and at 8MB or so a pop, things get unwieldy quickly.
  • A Google Earth KML file covering the whole world in 20° by 10° grid fields: Maidenhead_Locator_World_Grid. (If you’re feeling nerdy, here it is in Shapefile format: Maidenhead_Locator_World_Grid-shp).

If anyone would like their grid square in Google Earth format, let me know, or read on …

Making KML Files

Several people have asked, so here’s how you convert to KML. You’ll need the OGR toolkit installed, which comes in several open-source geo software bundles: FWTools/osgeo4w/QGis. Let’s assume we want to make the grid square ‘EN’.

    1. Run make_grid.pl:
      make_grid.pl en
    2. Convert to KML using ogr2ogr:
      ogr2ogr -f KML EN-maidenhead_grid.kml EN-maidenhead_grid.shp
    3. Alternatively, if you just want to extract a square (say EN82), you can use ogr2ogr’s ‘where’ clause to select just the geometry you want:
      ogr2ogr -f KML -where "Square='82'" EN82-maidenhead_grid.kml EN-maidenhead_grid.shp

 

using CSV as a virtual data source

While we already know how to make trivial shapefiles with shapelib, sometimes that’s too tedious. Very frequently I get data in Comma Separated Value (CSV) format, and reliably importing/converting it can be a pain.

Here’s our sample CSV file, library_test.csv:

"Easting","Northing","Library"
625539.6,4837170.9,"Dufferin St. Clair"
625862.0,4838241.1,"Oakwood Village"
626251.0,4835287.2,"Bloor Gladstone"
626671.7,4836922.6,"Davenport"
627227.2,4840006.4,"Forest Hill"

ogr has a CSV driver. In its documentation the Virtual Format driver is touched upon. This allows you to set up a data definition file, especially useful if you read the same format frequently.

Here’s the VRT file for that CSV:

<OGRVRTDataSource>
    <!-- note that OGRVRTLayer name must be basename of source file -->
    <OGRVRTLayer name="library_test">
        <SrcDataSource>library_test.csv</SrcDataSource>
        <GeometryType>wkbPoint</GeometryType>
        <!-- your SRS goes here; I used EPSG SRID -->
        <LayerSRS>EPSG:2958</LayerSRS>
        <GeometryField encoding="PointFromColumns" x="Easting" y="Northing"/>
   </OGRVRTLayer>
</OGRVRTDataSource>

Your CSV file will now behave like a shapefile, or indeed any other geo-format that OGR understands. QGIS is a bit picky – it doesn’t seem to always work out the path of the source file.

To prove these are real coordinates, here’s what I did to make a Google Earth KML file:

ogr2ogr -f KML -t_srs EPSG:4326 library_test.kml library_test.vrt -dsco NameField=Library

Technically, you don’t need to specify the SRS for KML output as it only supports EPSG:4326, but I found you got trivially different results if it was omitted.

Try this in Google Earth: library_test.kml