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Posted by chrisaaaaa on October 12, 2006, 7:42 am
Please log in for more thread options I can dig up the projection and datum, with this and the co-ordinates
of opposite corners do I have enough?
Chris
wrote:
> On 11 Oct 2006 16:56:31 -0700, chrisaa...@googlemail.com wrote:
>
> >I must be going mad, it cannot be this hard :)
>
> >I have some rectangular map bitmaps, I have the lat/long of upper-left
> >and bottom-right. Iwant to make these into GeoTiffs that will work with
> >my GPS/PDA.
>
> >There must be a program that makes the geotiff metadata file from this
> >information, I cannot believe I have to calculate the distance accross
> >the map and scale this to the number of pixels to enter into the
> >ModelPixelScaleTag.
>
> >Does anyone know of a little util to do this?
>
> >ChrisYour information is insufficient - you ned to know the projection of
> the image. If you scanned a map it is very unlikely to use x =
> longitude and y = latitude, as simply using latitude and longitude as
> linear coordinates gives unacceptable distortions. EVen Mercators
> projection, which uses axes the are parallel to latitude and longitude
> is scalled differentially with latitude in a complex manner (I'm
> afraid that this is a gross over-simplification!) The projection (and
> there are very many possibilities) is one thing you need to know to
> create the geotiff header. The other thing you need to know is the
> datum - in other words, what spatial framework the map uses. GPS, for
> example, uses WGS84, but maps may well use others, and the difference
> is quite substantial in the context of GPS - up to several hundred
> metres difference. Again, you need this information to create a
> geotiff.
>
> I understand there are utilities to create geotiff headers out there,
> but you need a lot more information than just the lat,long coordinates
> of the corner points of an image.
>
> Paul
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