street labeling

[Stephen Lime]

Nathan and all: I'll address symbol scaling and line labeling here. 

Symbol scaling requires 3 things:

  - map UNITS to be set
  - layer SYMBOLSCALE to be set
  - a scaleable symbol (only image symbols are not scalable)

UNITS are the units of a projection/coordinate system and are
needed to calculate scale. SYMBOLSCALE is the scale at which
a symbol or font should appear full size (eg. the defined size). Note
that only truetype fonts are scalable. Scaling works fine for shade
and marker symbols, but poorly for line symbols. About the only luck
I've had with scalable lines is using ellipse symbols for drawing stroked
lines (http://www.dnr.state.mn.us/fish_and_wildlife/trout_fishing/se_trout_init.html
is an example that uses scaled lines). Beyond that your best off using
multiple definitions of the layer valid at different scales and creating custom
line symbols for each. If anyone is aware of some good line scaling
algorithms let me know. Currently I'm working with the constraints of
the GD library.

There are two ways to label streets (or any lines for that matter) 
using MapServer. I'll describe  them using the more advanced control 
found in version 3.3.

 1) Labeling with text. MapServer can use text from a .dbf attribute to label streets
     with names. The label angles can be fixed, pulled from an item or calculated
     automatically. Rotated text is only available with truetype fonts. There are 
     several other parameters that allow you to fine tune label placement. All of these
     are properties of a label object.

     MINFEATURESIZE - given in pixels. Sets the smallest feature to be labeled. This
     can also be set to AUTO which forces labels to be the same size (width) as the
     segment they are labeling.

     MINDISTANCE - given in pixels. Sets the minimum distance between duplicate pixels.

     BUFFER - given in pixels. Reserves space around labels so text isn't so dense.

     OFFSET - x and y values given in pixels. Label offset relative to a horizontal feature. Rotation
     is taken into account.

     In general labeling with text can accompany the defintion of the line drawing. However, you can 
     also use it as annotation drawn later in the map. Here's an example layer from the contour labeling
     example at http://arachnid.dnr.state.mn.us/ms_test: 

LAYER
    NAME test
    TYPE LINE
    STATUS DEFAULT
    DATA "../../data/contours"
    LABELITEM "elev1"
    SYMBOLSCALE 10000
    LABELMAXSCALE 24000     
    CLASS
      COLOR 255 0 0
      LABEL
        TYPE TRUETYPE
        ANTIALIAS
        FONT times
        COLOR 0 0 0
        BACKGROUNDCOLOR 255 255 230
        SIZE 8
        MINSIZE 4
        MAXSIZE 8
        ANGLE AUTO
        POSITION CC
        MINFEATURESIZE 100
      END
    END
  END

 2) Labeling with text and a marker symbol. This is generally used for placing things
like highway symbols and numbers. You must use annotation layers to do this. The
label point is generated from a line shapefile, but only the label point is actually rendered.
The key here is that you get your label size to match your marker size. For the stuff I've
done the markers are GIF images created to match a certain font size. Rotation most
likey is useless here (marker is NOT rotated). Also since the marker is centered on the
label point a label POSITION of CC should be used. The BUFFER, MINDISTANCE and
MINFEATURE size options all apply here. Here's a sample from our recreation compass
(http://www.dnr.state.mn.us/compass): 

 LAYER
  NAME interstate
  STATUS DEFAULT
  DATA interstate
  TYPE annotation
  LABELITEM "road_name"  
  CLASS
    COLOR 255 255 255
    SYMBOL 1
    LABEL
      MINFEATURESIZE 50
      MINDISTANCE 150
      POSITION CC
      SIZE TINY
      COLOR 255 255 255
    END
  END
END # interstates

Annotation of this type is always drawn after the highway itself is drawn. I have a very small
collection of highway markers that I'll share if anybody wants them.

Steve

Stephen Lime
Internet Applications Analyst

Minnesota DNR
500 Lafayette Road
St. Paul, MN 55155
651-297-2937

>>> Nathan Carr <ncarr at guideguide.com> 11/23/99 07:49AM >>>
I am presently struggling to display arcs(roads in Lyon,Fr) with vector
symbol scaling 
and label orientation. Could somebody describe the definitions that are
needed for this.

For example how can I define a railway or motorway in the line symbol
file and have this
line and label scaled at different levels.
I have tried various combinations of the defs in the map file

Also the MAXSCALE and MINSCALE definitions seem to break the display 

Also should I make a layer up of points for displaying as an
'annotation' layer instead of labels.

Stephen Lime wrote: 
This is similar to how annotation layers are handles, but the difference
is with annotation the marker
 is drawn after any collision avoidance has been done so markers don't
overlap either.
-- 
Nathan Carr  ncarr at guideguide.com