TileMill wants to know: what projection is this data?

If you’re making maps, you will probably need to know **something** about cartographic projections. Here’s the minimum.

- The globe is round,
**maps** are flat. Each of the hundreds of different methods for converting from round to flat is a **projection**.
- When you have a latitude and longitude, you have
**unprojected coordinates**. Anything you can do with these doesn’t require choosing a projection.
- Most consumer web maps use the
**Web Mercator** projection, also known as the **Google Web Map** de facto standard, **EPSG:900913** (“google” written with numbers), **EPSG:3857**, etc.
- Government agencies, desktop apps and other stuff often use the
**WGS84** projection, also known as **EPSG:4326**.
- It is technically straightforward to
**convert** from unprojected coordinates to any projection, or between projections, using GIS packages or command line tools like **GDAL**. It can be slow to do this on the fly.
- Each projection is defined using a
**Spatial Reference System**. An SRS can also define systems of unprojected coordinates, and even other planets.
- There are half a dozen common
**formats** for describing the SRS, including:
- SRID, an identifier including the identifier scheme, like “EPSG:3857”, “ESRI:102113” or “SR-ORG:7483”.
- proj4, a short piece of text with lots of + and =, used by a tools like GDAL and TileMill. It looks like:
+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

- Well-known text (WKT), a verbose format that can also be used to define spatial data. For example:

GEOGCS[“GCS_Oman”,

DATUM[“Oman”,

SPHEROID[“Clarke_1880_RGS”,6378249.145,293.465]],

PRIMEM[“Greenwich”,0],

UNIT[“Degree”,0.017453292519943295],

AUTHORITY[“EPSG”,”37206″]]

- The
**tool** you are working with (eg, TileMill) will only **support certain projections**. You need to:
- Find data that is in the right projection (Web Mercator is the safest), or convert it; and
- Tell the tool what projection it’s in, if it can’t guess. You will have to pick from a list, or use one of the formats above, that it supports.

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