projection questions

[Ken Lord]

Hi Jeff,

Data that you are used to seeing as lat/long / geographic will appear
to be skewed in the manner you describe if you view it reprojected
into UTM or other projection systems and depending on where on the
earth it is located. ... It's not necessarily wrong, just different
and can take time to get used to.

There are three parameters effected by projections that make maps
appear very different ... shape of an object,  distance between
objects, and area of an object.  Generaly any particular projection
will handle 2 of the 3 well, so maps tend to be made using particular
projections for particular uses in particular regions.

Now if the earth were flat, we wouldn't have all these issues when
squishing 3d data onto 2d map images.

Cheers,
Ken Lord
Vancouver BC


On Fri, 21 Jan 2005 13:42:47 -0500, Jeff Portwine <jdport at veritime.com> wrote:
> Thanks for the explanation, that does make perfect sense.    I believe all
> of my shapefiles are NAD83... i'm not sure if they are all geographic but my
> guess is that they are.    So I guess my only question is why my map looks
> so ... skewed I guess is the only word I can come up with at the moment.  It
> just doesn't look right... the whole map seems almost twisted so that the
> coastline appears to be running more east/west than it should rather than
> north/south.  Perhaps my "image size" is set wrong so that it's compressing
> one axis and stretching another.... I will just have to play with it I
> guess.
> 
> Thanks for the great information.   If nothing else this mapping project has
> been educational.
> 
> -Jeff
> 
> ----- Original Message -----
> From: "Ed McNierney" <ed at topozone.com>
> To: "Jeff Portwine" <jdport at veritime.com>; <MAPSERVER-USERS at LISTS.UMN.EDU>
> Sent: Friday, January 21, 2005 12:56 PM
> Subject: RE: [UMN_MAPSERVER-USERS] projection questions
> 
> Jeff -
> 
> A datum is not a projection, but a mathematical model of the Earth.  Every
> geographic (i.e. latitude/longitude) and projected coordinate system is
> expressed relative to a datum.  Many people incorrectly think that the
> latitude and longitude of a point are absolute, unchangeable values.
> Latitude and longitude are geographic coordinates expressed relative to a
> datum.  If you change the datum, the coordinates will change.  In the United
> States, the NAD27 and NAD83 datums are commonly used.  WGS84 is also used
> because it is a world-wide datum, but in the USA you can consider it
> identical to NAD83.
> 
> For example, I'm typing this from a location that's latitude 42.5860°N,
> 71.5553°W in WGS84/NAD83 coordinates, and 42.5859°N, 71.5558°W in NAD27
> coordinates.  Not a big difference, but a real one.
> 
> If ALL of your shapefiles are geographic NAD83, things are pretty easy.  If
> your input and output projections are all the same (and let's start with
> that case) then you can completely ignore projections.  Just be sure the
> extents in your .mapfile are appropriate for the area and the UNITS are DD
> (decimal degrees).
> 
> It sounded, however, from your last email, as if you'd already had that
> working.  What works and doesn't work?  Can you get one shapefile to display
> using latitude/longitude extents for the northeastern US?
> 
>     - Ed
>