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Posted by Dave Patton on May 10, 2004, 12:40 am
Please log in for more thread options ejlucido@hotmail.com (Edward) wrote in
[top posting fixed]
>> ejlucido@hotmail.com (Edward) wrote in
>>
>> > Hi,
>> >
>> > Firstly, I have been searching this group for a how-to on
>> > georeferencing a raster image and/or creating a world file. I am
>> > still confused as to how to come up with the parameters that go
>> > into the world file. Can anyone give an easy to understand answer?
>> > I am a complete newbie in GIS and been assigned to develop a very
>> > basic GIS mapping program! I think my main confusion is what to do
>> > with the control points that the user enters? To me it looks like
>> > there is only one x,y coordinate that goes in the world
>> > file...please help!
>>
>> Do you know that there are all sorts of "mapping programs", including
>> free ones, that will let you georeference a raster image? Seems you
>> are "reinventing the wheel"?
>>
>> "FAQ: What is the format of the world file used for georeferencing
>> images?":
>> <http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.artic
>> leSho w&d=17489>
>>
>> Aside from considerations such as Datum and Projection, whether the
>> image is rectified and/or rotated, how many control points are
>> entered, etc., you use the control points to determine the scale
>> of the image(e.g. meters/pixel) and the coordinates of the upper
>> left pixel.
> Hi Dave
>
> Thanks for the reply. I am writing a GIS mapping application in
> Delphi, and am wanting to provide a very basic image registration
> feature. I am wanting to know how to determine the parameters that go
> into a world file from X number of control points (I want user to
> enter minimum of 3).
>
> Regards
> Edward
You might want to re-think your approach, and/or explain what it
is you are trying to do. For example, a world file, on it's own,
is insufficient to allow for geo-referencing a map image, because
it lacks any information about the datum, projection, map name,
map scale, etc.
As for how to calculate the world file values, given the values
of some control points, in the simple case it's just some basic
math, but to allow for 'all cases', you'll have to incorporate
logic to allow for re-projecting the map image. As I said,
re-inventing the wheel :-)
P.S.
Please don't top-post.
--
Dave Patton
Canadian Coordinator, Degree Confluence Project
http://www.confluence.org/
My website: http://members.shaw.ca/davepatton/
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