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The contents of a Map File are used by MapServer for configuration, data access, projection, and more. Because the Map File is parsed every time a map image is requested, it is important to think about what you include in the file in order to optimize performance. The optimal Map File is one that doesn’t include or reference anything that isn’t needed.
There are two ways to define projections in a Map File. You can either use inline projection parameters or specify an EPSG code for that projection. If you use the EPSG code method, PROJ looks up the projection parameters in the PROJ database using the EPSG code as an ID. This database lookup takes significantly more resources than when the projection parameters are defined inline. This lookup takes place for each projection definition using EPSG codes in a Map File.
Projection defined using inline projection parameters
PROJECTION "proj=utm" "ellps=GRS80" "datum=NAD83" "zone=15" "units=m" "north" "no_defs" END
Projection defined using EPSG Code
PROJECTION "init=epsg:26915" END
Use inline projection parameter definitions in place of EPSG codes.
If you want to use EPSG codes, remove all unneeded projection definition records from the PROJ EPSG database.
For every layer in a Map File that has a status of ON or DEFAULT, MapServer will load that layer and prepare it for display, even if that layer never gets displayed.
Build lean Map Files, only include layers that you plan to use.
Turn off unnecessary layers; the more layers MapServer is displaying, the more time it takes. Have your opening map view show only the minimum necessary to orient the user, and allow them to turn on additional layers as needed. This is particularly true of remote WMS or very large rasters.
Related to turning off layers, is turning them on but using MINSCALEDENOM and MAXSCALEDENOM to determine at what zoomlevels the layer is available. If a map’s display is outside of the layer’s MINSCALEDENOM and MAXSCALEDENOM range, then MapServer can skip processing that layer. It also makes for a really cool effect, that the national boundaries magically change to state boundaries.
If you have a complex application, consider using multiple simple and specific Map Files in place of one large “do everything” Map File.
In a similar vein, each class also supports MINSCALEDENOM and MAXSCALEDENOM. If your dataset has data that are relevant at different zoomlevels, then you may find this a very handy trick. For example, give a MINSCALEDENOM of 1:1000000, county roads a MINSCALEDENOM of 1:100000, and streets a MAXSCALEDENOM of 1:50000. You get the cool effect of new data magically appearing, but you don’t have MapServer trying to draw the nation’s roads when the entire nation is in view!
Classes are processed in order, and a feature is assigned to the first class that matches. So try placing the most commonly-used classes at the top of the class list, so MapServer doesn’t have to try as many classes before finding a match. For example, if you wanted to highlight the single state of Wyoming, you would probably do this:
CLASS EXPRESSION ('[NAME]' eq 'WY']) STYLE COLOR 255 0 0 END END CLASS STYLE COLOR 128 128 128 END END
But it would be a lot more efficient to do this, since 98% of cases will be matched on the first try:
CLASS EXPRESSION ('[NAME]' ne 'WY']) STYLE COLOR 128 128 128 END END CLASS STYLE COLOR 255 0 0 END END
Use tile indexes instead of multiple layers.
When the Map File is loaded, each raster symbol listed in the symbols file is located on the filesystem and loaded.
Only include raster symbols in your symbols file if you know that they will be used by your application.
To load a font, MapServer opens up the fonts.list FONTSET file which contains an alias for the font and the path for that font file. If you have a fonts.list file with a long list of fonts, it will take more time for MapServer to locate and load the font that you need.
Limit the entries in fonts.list to fonts that you actually use.