[MAPSERVER-USERS] Displaying population density with Mapserver

Wed May 28 13:34:44 EDT 2008

Faldor, 

When you have point coordinates stored in a text file, there really is
no way to encode/attach the projection definition.  Even if you could,
it doesn't sound like you really know what spatial reference system
(SRS) or projection that it is in.  The key to projecting to another SRS
or translating ground units to another measure (from degrees to meters)
is knowing what the current SRS of the data is.  

Since your data is worldwide and it is unprojected (lat/lon), a good
first guess would be to assume that it is unprojected with using the
WGS84.  

Try defining your layer projection by including this block in your layer
definition.

PROJECTION
   "init=epsg:4326"
END

David.

-----Original Message-----
From: mapserver-users-bounces at lists.osgeo.org
[mailto:mapserver-users-bounces at lists.osgeo.org] On Behalf Of
Pierre-Benoit Besse
Sent: Wednesday, May 28, 2008 4:11 AM
To: Fawcett, David
Cc: mapserver-users at lists.osgeo.org
Subject: Re: [MAPSERVER-USERS] Displaying population density with
Mapserver

Well, I confess, I'm new to GIS applications and I do not know a lot
about projections.
I understand I have to tell MapServer what is the projection of each
layer I use in the mapfile, but using ogrinfo on my shapefiles tells me
that :

Layer name: pop_density
Geometry: Point
Feature Count: 249937
Extent: (-180.000000, -56.000000) - (179.750000, 83.500000)
Layer SRS WKT:
(unknown)
latitude: Real (24.15)
longitude: Real (24.15)
densite: Real (24.15)

I generated this shapefile from a CSV file with 3 columns : latitude,
longitude, density_value.
Did I have to define a projection then ? How do I do that ?

Best regards,

Faldor

2008/5/27 Fawcett, David <David.Fawcett at state.mn.us>:

Have you defined an output projection for your map and input projections
for each of your layers?  It is likely that MapServer needs to know what
the input projection/SRS is before it can translate to different units.

David.
-----Original Message-----
From: Pierre-Benoit Besse [mailto:besse.pb at gmail.com] 
Sent: Tuesday, May 27, 2008 11:26 AM
To: Fawcett, David
Cc: Ritesh Ambastha; mapserver-users at lists.osgeo.org
Subject: Re: [MAPSERVER-USERS] Displaying population density with
Mapserver

Ha, well, I prefer that :) I would need a lot of layer definitions to
obtain something ok...

But I just tried the "ground units" stuff, I tried with a 50 kilometers
wide square and it drew squares the size of ASIA !! oO
I had to define a 1 feet wide square to obtain something more or less
ok...
Does anybody know why ?

Aside from that It seems to work, thank you !

I will try the scalesymbol stuff tomorrow.

Regards

2008/5/27 Fawcett, David <David.Fawcett at state.mn.us>:

Before you create lots of different layers to simulate symbol scaling,
you may want to try something else.  It has been a long time since I
have played with scaled symbols, but give it a try.

You should be able to scale the symbols by setting a SYMBOLSCALE in the
LAYER object.  This is the scale at which you want the symbol to appear
at the SIZE stated in the map file.

You may want to modify STYLE MAXSIZE if you think that you will want
your symbols to be larger than 50 pixels.

Another completely different approach that just came to my mind would be
to use ground units (e.g. meters) to set the size of your symbol.  To do
this, set LAYER SIZEUNITS to meters.  Calculate the size of a symbol in
meters on the ground that will touch adjacent symbols, or give you the
desired amount of space between the symbols.  That way, the symbols
should scale consistently.

David.

-----Original Message-----
From: mapserver-users-bounces at lists.osgeo.org

[mailto:mapserver-users-bounces at lists.osgeo.org] On Behalf Of Ritesh
Ambastha
Sent: Tuesday, May 27, 2008 10:16 AM
To: mapserver-users at lists.osgeo.org
Subject: Re: [MAPSERVER-USERS] Displaying population density with
Mapserver

Dear Faldor,

Robert is absolutely right. You will have to make different LAYERS for
the same POINT layer. But, define MAX/MINSCALE for each Layer. Change
STYLE -> SIZE values in each of the layer. You are done !!

Cheers !
Ambastha

Faldor wrote:
>
> Yes, but I need them to adapt for different extents.
>
> For example, When I zoom out enough to see the world, I need them 1
> pixel wide (it's ugly otherwise), but when I zoom in I will need then
> 2 or 5 or even 30 pixels wide to cover all space.
>
> 2008/5/27 Ritesh Ambastha <group.ritesh at gmail.com>:
>
>>
>> Dear Faldor,
>>
>> <snip>What I need is to have the points grow to cover all space
>> available when I zoom in.</snip>
>>
>> Do you mean that the size of points should grow? It can be done
>> easily by setting proper SIZE in the STYLE tag.
>>
>>
>> Regards,
>> Ambastha
>>
>>
>>
>> Faldor wrote:
>> >
>> > Hi,
>> >
>> > Sorry for my first empty message.
>> >
>> > My problem is, I need to display population density with Mapserver.

>> > The data comes from GPWv3 and has a resolution of 15'. I converted
>> > it to a POINT Shapefile using ogr2ogr.
>> >
>> > At first when I display it on a world map it shows nicely. But when

>> > I
>> zoom
>> > in it begins to show separated points for each value in the
>> > shapefile. What I need is to have the points grow to cover all
>> > space available
>> when
>> I
>> > zoom in.
>> >
>> > Is there any mean to do it with Mapserver ? I did not find it in
>> > the mapfile reference.
>> >
>> > I don't know if I explained it well (I'm not english speaking). If
>> > you
>> do
>> > not understand something, please ask.
>> >
>> > Cordialy,
>> >
>> > Faldor
>> >
>> >
>> >
>>
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