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Projected Data / Transformation Problem

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#1
Kartograph

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I seem to be too stupid for this :angry:

I've got a projected dataset, which uses a different ellipsoid than my target projection.
Namely, my set is a raster set in a projected coordinate system using the Bessel 1841 ellipsoid. How do I transform it into WGS84/UTM? Reprojections are no problem, but the underlying transformation needed really gives me headaches.

I tried many ways, but there always remains an offset, which I attribute to the different datums. So I figure I have to transform the projected into a geographic coordinate system, tranfsorm to geographic coordinates with a different ellipsoid (our beloved WGS 84), and then change into projected UTM.

I'm starting to grow grey hair, trying to do this in ArcGis, it seems you can only transform vector data.
Help and pointing out any flawed thinking greatly appreciated,

Andreas

#2
CHART

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Andreas,

To reproject raster look up BlueMarble Transformer program. Hans can elaborate (I think he is one of their distributer in Europe).

However it might not be a datum issue. You can calculate the offset in xy to figure that out. Also check the precision of the sources. It might just be that your two data sets don't match. Then maybe you will have to do a bit of local adjustments in the form of rubber sheeting (either the vector or the raster depending on the more precise source).
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#3
GISRox

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I'm not sure what tools you have at your disposal, but I'd suggest you take a look at the following:

Global Mapper - http://www.globalmapper.com/

Didger - http://www.goldensof...er/didger.shtml

Blue Marble - http://www.bluemarbl...gereproject.php

or

You could send me the file and I could do the transformation for you... :)



#4
Kartograph

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Thanks for the answers,

However it might not be a datum issue. You can calculate the offset in xy to figure that out. Also check the precision of the sources. It might just be that your two data sets don't match. Then maybe you will have to do a bit of local adjustments in the form of rubber sheeting (either the vector or the raster depending on the more precise source).


I?m quite sure it is a datum issue, as I tried to rubber sheet with another dataset, and it is at least a second order polynomal distortion. I need very high accuracy, so I?m uncomfortable with using the "rubbersheeted" results. Especially since I would have to fit large scale raster to center-line street data. I cannot accept a margin of error above half a meter.

BTW, you guys point me to other software. Does this imply that I?m corrct in assuming datum transformations with raster is not easily done with ArcGis?

I have access to global mapper, though my version doesn?t allow me define the projected coordinates system I need. Maybe it would be the smartest to ask the global mapper purveyor how to do that.

#5
GISRox

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I'm honestly not familiar enough with the capabilities of ArcGIS to do image re-projection and datum transformations. I have used some of the other products to do this type of work and I'm relatively confident that you need an application that canot only modify the image projection, but apply the datum transformation during the process.



#6
Hans van der Maarel

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Andreas,

As has been pointed out already, I am indeed a distributor for Blue Marble's Transformer, as well as Avenza's Geographic Imager (which uses the Blue Marble transformation libraries). Based on the bit of information you posted, I assume both products would be able to do the trick. Often the trick here is figure out the projection the data is in, but if you have correct metadata, that shouldn't be a problem.

Half a meter accuracy sounds pretty high though, what's your pixel size? :unsure:
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#7
Kartograph

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Well thanks, at least I am not feeling totally stupid anymore. :)
The ESRI help docs were very dodgy on that subject, so I assume I will have to use other software.
Oh, and I am sure there is an ArcGIS extension for doing this...

Half a meter accuracy sounds pretty high though, what's your pixel size?


~ 0.25m. Should do the trick, no?

Metadata are no problem, although the projected coordinate system has a wonky origin.

#8
Hans van der Maarel

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Half a meter accuracy sounds pretty high though, what's your pixel size?


~ 0.25m. Should do the trick, no?


Hmmm... that's very close. A 2-pixel margin of error doesn't seem like much when reprojecting.
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Red Geographics
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#9
GISRox

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So how did the re-projectiona and datum transformation go? What product did you end up using?



#10
pghardy

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I've got a projected dataset, which uses a different ellipsoid than my target projection.
Namely, my set is a raster set in a projected coordinate system using the Bessel 1841 ellipsoid. How do I transform it into WGS84/UTM? Reprojections are no problem, but the underlying transformation needed really gives me headaches.
I tried many ways, but there always remains an offset, which I attribute to the different datums. So I figure I have to transform the projected into a geographic coordinate system, tranfsorm to geographic coordinates with a different ellipsoid (our beloved WGS 84), and then change into projected UTM.
I'm starting to grow grey hair, trying to do this in ArcGis, it seems you can only transform vector data.
Help and pointing out any flawed thinking greatly appreciated,


You may be having problems because Bessel 1841 is an ellipsoid, not a datum (geographic coordinate system). Several different datums can use the same ellipsoid. We therefore don't have any transformations to convert between Bessel 1841 and WGS84. If you can identify what location the data represents we may be able to identify what datum is probably being used. Then you could redefine the data's coordinate system and try projecting it on-the-fly.

Prior to ArcGIS 9.2, the raster project tool didn't support datum transformations, but there are tools available in ArcScripts (http://arcscripts.esri.com) that remedy the problem.

Drop me a direct email with more information about the data if you want us to look into this.
--
Paul Hardy
ESRI Europe (phardy@esri.com)




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