Layer priority

[Mark Presling]

Hi all.

I'm new to the list and a relative newbie to MapServer.

I'm having trouble with label priority... I have 3 layers - city, town, 
populated place and the populated place labels always render in 
preference to towns and cities, but I want city labels to always render 
with a higher priority. That is, if MapServer has to decide whether to 
render a pp label or a city label when they are in the same place I want 
the city one to be rendered.

All I can find on this subject is an email from Stephen Lime in 2001 
that said this:
"Labels are placed into the cache as layers are processed- they have to 
be, and then the
cache is processed in reverse order so that features in the last layer 
drawn (i.e. the top layer)
are labeled first. I've not had problems with it."

However, changing the order of layers in the mapfile doesn't fix it. I'm 
using WMS to access it, if that makes any difference. Here is what was 
said on IRC this morning:

<bitnerd> mrp: try playing with labelcache and postlabelcache in your 
layer definition you can do a lot to tweak labeling behaviours with those
<bitnerd> http://mapserver.gis.umn.edu/docs/reference/mapfile/layer
<mrp> Thanks bitnerd. I have tried these, *but I really want the label 
cache to work as normal so that I don't get overlaps and clutter.* The 
labels can be quite high density. I just need to be able to prioritise 
the order they are rendered, but the order in the mapfile doesn't seem 
to help
<FrankW> mrp: Did you change the order of your layers in the .map file 
or in the WMS request?
<FrankW> I'm not sure, but the order of layers in the WMS request may be 
significant.
<mrp> I am using uDig to access the WMS. Changing the order in .map and 
uDig don't seem to make a difference
<FrankW> If you can see the apache log file, check the order of layers 
in the action wms and see if they vary.
<mrp> ok, I'll have a look now
<mrp> 
LAYERS=land,nmcregion,exotic,forest,lake,native,railway,river,riverbed,road,rsrp,town,populatedplace,city
<mrp> 
LAYERS=land,nmcregion,exotic,forest,lake,native,railway,river,riverbed,road,rsrp,town,city,populatedplace
<mrp> That's the WMS requests... the order of city and pp makes no 
difference
<FrankW> Hmm, I'm out of ideas.
<mrp> they are currently ordered in the mapfile (top to bottom) as city, 
town, pp. Changing that order around seems to make no difference

I just need to know what things I can try or look out for (and what the 
expected behaviour is).


This is with all 3 layers turned on but with "POSTLABELCACHE true" set 
on the city and town layers:
[Damn, the mailing list email size limit is too small for a single 
screenshot!!! That's not so good is it??]

I would expect that because city is at the top of the list the labels 
would be rendered in the same order of priority.

Thanks,
Mark



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