md `## Working with geodata We need to load our geodata in file formats, data formats and coordinate systems that d3 suppports. The file format natively supported by D3 is [GeoJSON](http://geojson.org/), so I'll use a GeoJSON file containing all the disbaility parking spots in Vancouver (my hometown) I downloaded from the Vancouver Open Data catalogue.
But geodata are more complicated than just the file format!

Geodata are coded using

a geodetic coordinate system format called a "datum" as a spatial reference;
and a projection.

Datum: Let's talk about the "datum" format first (DO NOT mix this up with the convention of referring to the current object in Vega or d3 as a \`\`\`datum\`\`\`: the d3 \`\`\`datum\`\`\` is quite different.) These all have a particular specification ID that you can find at http://spatialreference.org/. The standard longitude and latitude references we use in d3, as far as I can tell, are WGS84 format - EPSG:4326. HOwever, many geodata come in different coordinate systems: Census Canada, for example, uses Lambert Conformal Conic. The City of Vancouver uses EPSG:26910. The geodetic coordinate system used in the odata you have should be listed in an accompanying file (in the .prj file if the original data format is a shapefile directory). So you have to need to find the coordinate system (there may or may not be an EPSG number), and then convert the coordinates from the original form to the standard EPSG:4326(lat,long). There are a number of ways to do this. You can download the free qGIS tool (if you want to dive into the full spectrum of GIS and geodata) and export the layers to the apporpriate datum in the LAyers menu. You can also use the arcGIS tools if you have access to them (it's an expensive license, but most universities have full access). Or if you only have a couple of coordinates you can use an online pair conversion tool like https://mygeodata.cloud/cs2cs/.

Projection: d3 (and by extension the Vega and Vega-Lite grammars) supports many projections (take a look at them in this workbook: https://observablehq.com/@vega/vega-lite-cartographic-projections). We can typically use the \`\`\`Mercator\`\`\` projection for most cases. But you will probably need to play with the right scaling factor and centering to position your map as you want it. Here is another VERY POORLY documented fact: a d3 geoProjection takes coordinates in an [x,y] or [long,lat] format. In other words, longitude and latitude are switched from the normal way they will be expressed in many datasets - even after you have done the datum conversion. Typically the most useful workaround is simply to rename your data fields. Urk.`