[mapserver-users] Dual role of LAYER is confusing

[Rahkonen Jukka (Tike)]

Hi,

In most cases in Mapfile LAYER is defining the output of the service. However, it looks like the original way to use DATA has not been flexible enough to suit new sources of data and as a workaround one layer is used as an input for another layer. For example if a shapefile is used as tileindex it is enough to write  TILEINDEX "tiger/index.shp" 
However, if one wants to take tileindex from Spatialite it must be done by defining a layer first

LAYER  # The tileindex layer
        NAME "spatialite_tileindex"
        STATUS OFF
        TYPE POLYGON
        CONNECTIONTYPE OGR
        CONNECTION "/orthophotoindex.sqlite"
        DATA "select * from orthophotos where year=2013"
       PROJECTION
          "init=epsg:3067" 
        END
END # End of tileindex layer
LAYER # The orthophoto layer
        NAME "orthophotos_2013 "
        STATUS ON
        TILEINDEX "spatialite_tileindex"  # Name of the tileindex layer
....

I guarantee that many if not all new Mapserver users consider this as tricky. However, situation can be even more tricky. The new RFC about heatmaps/density maps gives an example http://mapserver.org/development/rfc/ms-rfc-108.html.
The source data for the heatmap layer is configured as

LAYER
    NAME "heatmap"
    TYPE raster
    CONNECTIONTYPE kerneldensity
    CONNECTION "points"

and here "points" is referring to a layer.  It the "points" layer is using ogrtileindex where tileindex is not a shapefile but comes from a database we will need three layers for publishing the heatmap:
- ogrtileindex layer
- "points" layer
- heatmap layer

This is doable but confusing. What is also confusing is that I have not yet found a way to hide those technical layers from WMS.

Until now after making a web search during a coffee break, it is naturally done with a layer level metadata item "ows_enable_request" "!*" as described in http://mapserver.org/de/development/rfc/ms-rfc-67.html.  Great. But still I feel the dual role of "layer" a bit confusing.  At least in the tileindex case I wish I could simply write just one line even if with somehow complicated syntax. Perhaps it could be something like

TILEINDEX "SELECT * from orthophotos where year=2013 USING OGR CONNECTION /orthophotoindex.sqlite"

-Jukka Rahkonen-