Jump to content

 
Photo

Transform Coordinate System with Avenza Geographic Imager

- - - - -

  • Please log in to reply
4 replies to this topic

#1
Crestfallen

Crestfallen

    Newbie

  • Validated Member
  • Pip
  • 1 posts
  • United States

For the record, I'm a cartographic novice. I'm familiar with Adobe Illustrator and Photoshop from graphic design projects but have had little reason to dip into map making until being commissioned to make several maps for an academic project at my university.

I'm using the free map dataset from Natural Earth. I'm editing the vectors in Illustrator with Avenza MAPublisher. It's my understanding that the Natural Earth data is projected in geodetic WGS84 by default (at the very least, this is the coordinate system automatically specified when importing the data to Illustrator with MAPublisher).

To create a scale bar in MAPublisher, the coordinate system must be projected; therefore, I've transformed the data from geodetic to a projected coordinate system (specifically, Winkel Tripel). So far so good.

I want to include a raster shaded relief layer in the final maps. For this, MAPublisher allows the import of raster images through the Register Image filter. However, MAPublisher does not have the capability to transform raster images from one coordinate system to another. The user guide suggests using Avenza's Geographic Imager add-on for Photoshop to transform the imagery to match the coordinate system of the data in MAPublisher before importing it (in my case, transforming it from the geodetic WGS84 default to a projected coordinate system - Winkel Tripel - which would allow for the implementation of scale bars).

For this, I'm using the grayscale Natural Earth shaded relief. When opening the file (.tif) in Photoshop, Geographic Imager automatically detects the associated reference file (.tfw). The coordinate system has to be manual specified, so I've selected geodetic WGS84 (screenshot). Everything appears fine until I attempt to transform the data to a projected coordinate system (Winkel Tripel or any other). Geographic Imager goes through the lengthy process of transforming the image data, yet, once the process is complete, the canvas size has changed but the data has disappeared, leaving only the checkered transparent background (screenshot).

I don't know what I'm doing wrong. Perhaps the Natural Earth data isn't geodetic (in degrees) after all. In this case, beginning with the wrong coordinate system might explain the disappearance of the data after transforming it in Geographic Imager. If I begin the process in Geographic Imager by specifying the coordinate system as projected (rather than geodetic) and then transform to a different projected coordinate system, the data doesn't disappear as before. However, the resulting projection doesn't correspond with the same projection of the vector data in Illustrator via MAPublisher. In any case, I'm fairly certain that the Natural Earth data is not projected by default since MAPublisher forces me to change the vector data from geodetic to projected before allowing scale bars, so I'm at a complete loss.

Any help would be appreciated.

#2
Hans van der Maarel

Hans van der Maarel

    CartoTalk Editor-in-Chief

  • Admin
  • PipPipPipPipPipPipPip
  • 3,859 posts
  • Gender:Male
  • Location:The Netherlands
  • Interests:Cartography, GIS, history, popular science, music.
  • Netherlands

There were some errors with the tfw files that were supplied with Natural Earth. I produced some new ones and Tom said he'd update the site over the holidays. With the updated world files I've been able to transform Natural Earth raster data within Geographic Imager without any problem (it still takes time of course). Which size/resolution imagery are you trying to use?
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#3
Nick H

Nick H

    Legendary Contributor

  • Validated Member
  • PipPipPipPipPip
  • 307 posts
  • Gender:Male
  • Location:Caversham, Reading, England.
  • United Kingdom

Are the new world files in place Hans? I've just downloaded the medium size version, extracted the part that covers the UK and warped this to OSGB with no problems.

Regards, N.
Caversham, Reading, England.

#4
Hans van der Maarel

Hans van der Maarel

    CartoTalk Editor-in-Chief

  • Admin
  • PipPipPipPipPipPipPip
  • 3,859 posts
  • Gender:Male
  • Location:The Netherlands
  • Interests:Cartography, GIS, history, popular science, music.
  • Netherlands

Are the new world files in place Hans? I've just downloaded the medium size version, extracted the part that covers the UK and projected this as OSGB with no problems.


I'm not sure, but I suspect that the error that was in there has little effect on areas that don't include any of the edges of the file.

Anyway, for completeness sake:
10M hi-res
0.01666666666667 0.00000000000000 0.00000000000000 -0.01666666666667 -179.99166666666667 89.99166666666667
10M lo-res
0.02222222222222 0.00000000000000 0.00000000000000 -0.02222222222222 -179.98888888888889 89.98888888888889
50M
0.03333333333333 0.00000000000000 0.00000000000000 -0.03333333333333 -179.98333333333333 89.98333333333333
Open an existing tfw file with a text editor, replace its content with the appropriate updated version and save it again under the same name. Save it as a plain text file. The tfw file should contain only 6 lines, nothing else.

For those of you who are interested in how it works... Line 1 is the x dimension of a pixel, line 4 is the negative y dimension of a pixel. Lines 5 and 6 are the coordinates of the center of the top-left pixel (Tom erroneously put the corner coordinates in there). You can check for yourself: Line 5 should be -180 + (0.5 * line 1), line 6 should be 90 + (0.5 * line 4)
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#5
Nick H

Nick H

    Legendary Contributor

  • Validated Member
  • PipPipPipPipPip
  • 307 posts
  • Gender:Male
  • Location:Caversham, Reading, England.
  • United Kingdom

Anyway, for completeness sake: ...

10M lo-res

0.02222222222222 0.00000000000000 0.00000000000000 -0.02222222222222 -179.98888888888889 89.98888888888889
Open an existing tfw file with a text editor, replace its content with the appropriate updated version and save it again under the same name. Save it as a plain text file. The tfw file should contain only 6 lines, nothing else.For those of you who are interested in how it works... Line 1 is the x dimension of a pixel, line 4 is the negative y dimension of a pixel. Lines 5 and 6 are the coordinates of the center of the top-left pixel (Tom erroneously put the corner coordinates in there). You can check for yourself: Line 5 should be -180 + (0.5 * line 1), line 6 should be 90 + (0.5 * line 4)

Ah, all is made clear. At present the tfw for this file is still as shown below.
0.02222222222222 0.00000000000000 0.00000000000000 -0.02222222222222 -180.00000000000000 90.00000000000000
I guess the image is a tiff with a world file rather than a geotiff so that it can be manipulated in graphics software without losing metadata.

Many thanks for the explanation.

Regards, N.
Caversham, Reading, England.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

-->