Projection/Extent help

Thu Dec 16 10:08:32 PST 2004

Paul -

First, ALWAYS reply to the entire list.  Just because I was the first to
answer your question doesn't mean I'm the best to answer the rest of
your questions, nor am I agreeing to be your personal answer man for
life <g>.  And it's important for everyone else to learn from your
questions, and to have them go into the archives.  Thanks!

Well, I guessed wrong - you didn't say whether the layer in your map
file was the DD layer or the FEET layer.  And I overlooked the
PROJECTION block in your LAYER definition - sorry about that; I didn't
scroll down far enough <g>.

Is 26971 incorrect?  Well, it's units are not feet, but meters, as are
most state plane systems.  Get rid of the UNITS FEET in the LAYER block,
as it's not going to change your projection for you.  Your coordinates
appear to be 26971 coordinates converted to feet, so you will need to
create a custom projection definition that's the equivalent of NAD83
Illinois East in feet.  Here's the definition of 26971:

NAD83 / Illinois East
  <26971>
    proj=tmerc 
    lat_0=36.666666667 
    lon_0=-88.333333333 
    k=0.999975 
    x_0=300000.000 
    y_0=0.000 
    ellps=GRS80 
    datum=NAD83 
    units=m 
    no_defs 
  <>

If you replace the "units=m" statement with "to_meter=0.3048006096" you
will adjust this projection definition to be in feet.  You will need to
explicitly describe this projection as:

PROJECTION
    "proj=tmerc"
    "lat_0=36.666666667"
    "lon_0=-88.333333333"
    "k=0.999975"
    "x_0=300000.000"
    "y_0=0.000"
    "ellps=GRS80"
    "datum=NAD83"
    "to_meter=0.3048006096"
    "no_defs"
END

This should accurately describe your data layer, and allow MapServer to
reproject it.

        - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: Camden Daily [mailto:cdaily at gmail.com] 
Sent: Thursday, December 16, 2004 12:15 PM
To: Ed McNierney
Subject: Re: [UMN_MAPSERVER-USERS] Projection/Extent help

Ed,

Thanks for the reply, but I'm still a little confused...

> Your EXTENT and UNITS need to be described in the units of the output 
> projection your map is using.  In your example below, everything is 
> fine.  But if you switch to the two commented EXTENT and UNITS lines, 
> it won't work unless your PROJECTION block is changed to have the map 
> generated in a different projection that uses those units.

The example I posted doesn't work right now.  In fact, the only I way I
can get it to work is to drop out the mapfile projection and switch to
the commented out extent in feet, the same units as the layer.

> Leave your EXTENT and UNITS alone, since you want to (I think) still 
> have your output projection be "latlong".  You need to create a new 
> PROJECTION block that's *inside* the LAYER definition block for the 
> new layer that's got units in feet.

I have a projection block within my layer.  Is it incorrect?

> By default MapServer assumes that all LAYERs not otherwise described 
> are in the same projection as the output projection of the image (the 
> one defined by the top-level PROJECTION block).  If that's not the 
> case, MapServer can't guess - you have to tell it that a given layer 
> is in a different projection by including a PROJECTION block inside 
> that layer definition.
> 
> What is the "other" projection?  You need to know either its name (if 
> it's a common, standard projection) or all the mathematical parameters

> describing it if it's a custom projection.  If you don't know how to 
> describe it accurately, MapServer won't be able to guess for you <g>!

The other projection that I'm trying to merge this with is in latlong,
which is why I'm trying to get this layer working with the extent in dd
as opposed to feet.

Thanks,
-Camden

>         - Ed
> 
> Ed McNierney
> President and Chief Mapmaker
> TopoZone.com / Maps a la carte, Inc.
> 73 Princeton Street, Suite 305
> North Chelmsford, MA  01863
> ed at topozone.com
> (978) 251-4242
> 
> -----Original Message-----
> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] 
> On Behalf Of Camden Daily
> Sent: Thursday, December 16, 2004 11:30 AM
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: [UMN_MAPSERVER-USERS] Projection/Extent help
> 
> Hello all.  I'm a mapserver newbie trying to figure something out.
> Basically, I have two sets of shapefiles in different projections and 
> units.  One has units of dd, and one has units of feet.
> 
> When I combine the two, I'd like to define my extents for the whole 
> .map file using dd instead of feet.  I've made up a sample .map file 
> where I'm simply trying to display one layer with units of feet while 
> defining the extent using dd.  However, everything I try just gives me

> a blank page.
> 
> If I define the extent in terms of feet and drop the "proj=latlong"
> line, it shows up fine.  But I just can't get it to work when I define

> the extent in dd.
> 
> Any help would be much appreciated!
> -Camden Daily
> 
> MAP
> 
>   NAME Chicago
>   STATUS ON
>   DEBUG ON
>   SIZE 300 300
>   IMAGETYPE PNG
>   IMAGECOLOR 240 240 240
>   SHAPEPATH "/web_system/data"
>   FONTSET "/web_system/fonts/fonts.list"
> --------
>   EXTENT -87.74 41.80 -87.56 41.98
>   UNITS DD
>   PROJECTION
>     "proj=latlong"
>   END
> -------
>   #EXTENT 1101584.450000 1841973.900000 1186739.850000 1969991.020000
>   #UNITS FEET
> -------
>   WEB
>     LOG "/web_system/logs/mapserver.log"
>     IMAGEPATH "/web_system/tmp/"
>     IMAGEURL "/tmp/"
>   END
> 
>   LAYER
>     NAME "cta_line"
>     TYPE LINE
>     STATUS DEFAULT
>     DEBUG ON
>     DATA "CTA_Line"
>     CLASS
>       COLOR 0 0 0
>     END
>     PROJECTION
>        "init=epsg:26971"
>     END
>     UNITS FEET
>   END
> 
> END
>