newb - calc'ing geotiff world file internals

Ed McNierney ed at TOPOZONE.COM
Mon May 9 08:07:22 EDT 2005


Mike -

Your first email suggested a calculation based on coordinates of the
image corners.  That will require you to understand the details of the
projection, as I mentioned in my first email.  Getting UTM coordinates
of the corners of a map that's not in UTM projection won't help you
project it correctly.

If by "standard USGS topo maps" you mean USGS DRG scanned map images,
you may have an easier job of it.  Unfortunately, California did their
own thing in creating the scanned DRGs.  It's the ONLY state where the
standard DRG map projection is NOT UTM.  It's an Albers projection
created just for the purpose.  And then California stopped dealing with
the updates, so any updated DRGs were produced by the USGS in their UTM
standard, so you may actually end up with a mix of map types.

There are several firms (TopoZone is one) who sell "standardized"
topographic maps, so you can get a single, seamless, UTM-projected image
of the area you want.  You may not be able to get the exact same map
image you're using now, so you should look around and decide whether
that alternative is useful to you.

     - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com 

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Mike D'Ambrogia
Sent: Monday, May 09, 2005 12:23 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] newb - calc'ing geotiff world file
internals

You know, I could just get std USGS topo maps of the same area and build
the tiles off the topo maps, would that make the projection calc easier?

Mike

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Mike D'Ambrogia
Sent: Sunday, May 08, 2005 8:56 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] newb - calc'ing geotiff world file
internals


Ed,

Thanks for the response

Was thinking about this while driving around awhile ago, if I know the
UTM or lat/long of each corner of each image couldn't I determine the
distance across the map in meters or feet and then divide that by 673 to
get pixel X value?  And then do the same for the top to bottom of each
image and divide by 795 to come up with a pixel Y unit value?

>>You can relax things a bit if you're simply trying to display the
data. If you
>>don't really care about coordinates, AND if you don't intend to
integrate other
>>data sets with this one, you can just make things up.

I do want to map data to coordinates, but I think I'll roll right now
with your logic below just to see if I can get the raster data into the
system and have it display correctly.  Something to build on in the
meanwhile

Thanks for the reply

Mike



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