Layers are how sets of data are added to a map in TileMill. Each layer references a single file or database query. Multiple layers can be combined over top of each other to create the final map - if you are familiar with layers in Photoshop or other graphics software the concept is very similar.
Layer types supported by TileMill
TileMill currently supports creating map layer from the following types of files and databases.
TileMill can plot points using comma separated value (CSV) files if they contain columns for latitude and longitude. The file’s first row must contain headings for the columns that contain the coordinates. Any of the following column headings are acceptable:
Usually referred to simply as ‘shapefile’, this is de facto standard file format for portable GIS data storage and is supported by most GIS software applications. The tricky part is that a single ‘shapefile’ is actually a collection of at least three files (possibly more). Common components are:
file.shp(required) contains the geographical point, line, or polygon information
file.dbf(required) a database of information describing objects in the .shp file
file.shx(required) an index file
file.prj(recommended) contains information about the projection the data is stored in. If this file is not present you cannot use ‘autodetect’ as your SRS choice.
file.index(recommended) is an alternative index format for TileMill’s renderer, Mapnik, that can speed up rendering significantly. Generate it with the command-line utility ‘shapeindex’.
If your shapefile datasource is an HTTP URL, the various corresponding files must be stored together in a zip file before they can be added to a TileMill project. For local datasources you may select just the .shp file and TileMill will find the appropriate corresponding files as long as they are in the same directory as the .shp.
KML is a standard geospatial data format that was popularized by Google with the Google Earth and Google Maps products. TileMill has limited support of KML – embedded styles will be ignored, and other features such as images, flythroughs, and 3D models are not supported. There is also no support for the compressed KMZ format at this time: to use a KMZ file, simply uncompress it as a zip file, and then use the resulting KML file.
GeoTIFF is a popular format for storing geospatial raster imagery such as satellite photography, remote sensing imagery, and digital elevation models.
Since Mapnik is currently unable to reproject raster data sources, to load them in TileMill you must ensure they are in Web Mercator projection. This can be done using the
gdalwarp tool. For example, to reproject a Natural Earth world geotiff from its native WGS84, you would use a command such as
gdalwarp -s_srs EPSG:4326 -t_srs "+proj=merc +a=6378137 +b=6378137 \ +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null \ +wktext +no_defs" -r cubic \ -te -20037508.34 -20037508.34 20037508.34 20037508.34 \ input_file.tif output_file.tif
SQLite is a compact database format with spatial types - and Spatialite is an extension to SQLite that provides PostGIS-like functionality, including reprojection. TileMill supports both formats through the ‘SQLite’ panel of the Add Layer dialog.
PostGIS is an extension for the general-purpose PostgreSQL database that allows you to store geographical objects in a database. It provides special functions and indexes for querying and manipulating spatial data and can be used as a powerful storage/analysis tool. From TileMill you can connect to a PostGIS supported database and run queries from the application directly.
To add a new layer to a TileMill project, click on the ‘Add Layer’ button at the top of the layers list. A dialog will appear prompting you for details about your layer.
This will be the name of the layer. It can be whatever you want, but must contain only letters, numbers, dashes, or underscores, and be unique within the project (two layers can’t have the same ID, even if they are the same file). A good ID is a short, accurate description of the layer’s purpose in a design, for example, ‘major-roads’, ‘state-borders’, or ‘lakes’. This setting is required for all layers.
These can be thought of as tags with which to classify your layers. You may have 0 or more separated by spaces, and the same character limitations as IDs apply. Multiple layers may have the same class - in fact, their usefulness is in allowing you to select multiple layers at once using a single class reference in your stylesheet.
Required for all layer types except for PostGIS, where it does not apply.
This is a path to a geospatial file either on your local filesystem or as a public HTTP URL. Local files may be selected from the filesystem by clicking ‘Browse’.
Required for PostGIS layers. Does not apply to any other layer type.
This field sets up various information required to connect to a PostgreSQL database. The following options are available:
dbnameRequired. The database that containing geodata for your layer.
userYour PostGIS username.
passwordYour PostGIS password.
hostThe hostname to use when connecting to your server.
portThe port to use when connecting to your server.
Here are some example connection strings:
dbname=gis dbname=gis user=mapbox dbname=gis user=mapbox password=foo host=localhost port=5984
This is the spatial reference system your datasource is stored in. TileMill can try to autodetect this value for certain types of files.
Table or subquery
Required for PostGIS and SQLite layers. Does not apply to other file-based layer types.
Databases contain one or many database tables, and TileMill needs to know which of these tables to pull data out of. Unlike non-database sources, you also have the power to add particular subsets of your or even make temporary adjustments to the data. For this TileMill has a Table or subquery field. To add an entire table from a database, simply enter the table name in this field. To specify a subquery it must be wrapped in parentheses and be given an alias using the
AS statement. For example:
(SELECT * FROM geodata WHERE type = 'birdhouse') AS data
Unique Key Field
This setting only applies to PostGIS layers and is optional.
If you plan to use the tooltip feature in TileMill, your PostGIS table must provide a column that contains a unique key. Specify the name of that column in Unique key field box. Otherwise, you may see incorrect tooltips after exporting to MBTiles.
This setting only applies to PostGIS layers and is optional.
TileMill can usually determine which field contains your layer’s geometry, but if it can’t you can explicitly set it here.