md `## Working with geodata We need to load our geodata in file formats, data formats and coordinate systems that d3 suppports. The file formats natively supported by D3 are [GeoJSON](http://geojson.org/) and TopoJson along with vanilla csv.I'll discuss each and use examples from a GeoJSON file containing all the disability parking spots in Vancouver (my hometown) I downloaded from the Vancouver Open Data catalogue.
But geodata are more complicated than just the file format!
To understand how mapping data work, we need to briefly identify key concepts that arise when we take a location on a spheroid (the Earth) and transform it into a 2D location on a plane (our maps). We call this kind of spatial data geodata and deriving them is a fundamental element in Geographic Information Systems, or GIS. Geodata are coded according to:

a Geographic Coordinate System format (GCS), (sometimes called a Coordinate Reference System (CRS), is a reference framework that defines the locations of features on a model of the earth. It’s shaped like a globe—spherical. Its units are angular, usually degrees. A GCS uniquely defines all locations on Earth with coordinates. The World Geodetic System, or WGS , is currently the standard for spatial reference, based on a vast network of satellites.
A geodetic datum is the part of the GCS that determines which model (spheroid) is used to represent the earth’s surface and where it is positioned relative to the surface. Since the earth’s surface is not perfectly smooth or round, there are many different datums designed for different parts of the world. Its units are in meters, as distance from a reference pont.
A Projected Coordinate System (PCS) contains a GCS and converts it into a flat surface, using a projection algorithm and other parameters. A projection\ is the mathematical algorithm that defines how to present the round earth on a flat map. Its resulting units are coordinates in *latitude* and *longitude*.

In short,

A GCS defines where the data is located on the earth’s surface.
The datum defines the actual position on the spheroid.
A PCS tells the data how to draw on a flat surface, like on a paper map or a computer screen, and transforms that relative position from the datum into coordinates of latitude and longitude.

### Why do you need to know all this? Isn't it automatic? Well no, it's not. Because these datums were designed for different parts of the world, and different degrees of precision and projection, it's very common to have to transform your data from one coordinate system to another. Or you may find a location that you thought should be in Vancouver is showing up on the other side of the world! In particular, you need to explicilty verify that the whatever tool or code you are using has the right datum and projection. d3 expects the WGS84 datum, but can work with a number of different projections. #### Datum Let's talk about the datum first. Each is assigned an EPSG code. GIS systems and libraries use these EPSG definition data for identifying coordinate reference systems, projections, and performing transformations between these systems. The EPSG code is a specification ID that you can find at http://spatialreference.org/. The standard longitude and latitude references we use in d3, for example, use the EPSG:4326 datum. (This is commonly called WGS84). HOwever, many geodata come in different coordinate systems: Census Canada, for example, uses Lambert Conformal Conic (EPSG:3347). The City of Vancouver uses EPSG:26910. The geodetic coordinate system used in the data you have should be listed in an accompanying file (in the .prj file if the original data format is a shapefile directory). So you 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 what d3 expects: EPSG:4326.
The bad news is that sometimes you will need to wrangle these data into the right PCS. The good news is that there are a number of easy ways to do this.

You can download the free qGIS tool (http://www.qgis.org) (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. ### More on Mapping, GIS and geodata There are many great resources if you're interested. I drew from some of these:

the ESRI ArcGIS blog https://www.esri.com/arcgis-blog/products/arcgis-pro/mapping/coordinate-systems-difference/#datum);
qGIS guides https://docs.qgis.org/3.16/en/docs/
Google's geocoding API docs https://developers.google.com/maps/documentation/geocoding/overview

Finally - 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.`