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Birthright Interactive Map - Raster manipulation README

This README is about raster file manipulation and use.

How to georeference a plain jpeg image

I used the [World File] 1 method, which requires knowing how many map units there is in 1 pixel. Here's how I calculated it.

The [Map of Cerilia original JPEG file] 7 is 5032px X 3151px. Using Gimp, I selected the scale within the map, which gave 75miles for approximatively 225px. Converting that gave 1 mile / 3 pixels, or 1 pixel / 533.33 meters. I chose 'meters' as my map units rather than miles.

Following the [World File] 1 definition, I created a .jpw file with the following content:

533.333333
0.0
0.0
-533.333333
0.0
0.0

When the file gets the same file name as the original jpg file (minus the extension), you can use [ogrinfo] 3 utility program to get its extent :

adube@weltall:~/proj/birthright-map/data$ gdalinfo cerilia-original.jpg
Driver: JPEG/JPEG JFIF
Files: cerilia-original.jpg
       cerilia-original.jpw
Size is 5032, 3151
Coordinate System is `'
Origin = (-266.666666500000019,266.666666500000019)
Pixel Size = (533.333333000000039,-533.333333000000039)
Metadata:
  EXIF_ColorSpace=65535
  EXIF_DateTime=2006:02:12 20:59:21
  EXIF_Orientation=1
  EXIF_PixelXDimension=5032
  EXIF_PixelYDimension=3151
  EXIF_ResolutionUnit=2
  EXIF_Software=Adobe Photoshop 7.0
  EXIF_XResolution=(72)
  EXIF_YResolution=(72)
Image Structure Metadata:
  COMPRESSION=JPEG
  INTERLEAVE=PIXEL
  SOURCE_COLOR_SPACE=YCbCr
Corner Coordinates:
Upper Left  (    -266.667,     266.667) 
Lower Left  (    -266.667,-1680266.666) 
Upper Right ( 2683466.665,     266.667) 
Lower Right ( 2683466.665,-1680266.666) 
Center      ( 1341599.999, -839999.999) 
Band 1 Block=5032x1 Type=Byte, ColorInterp=Red
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 2 Block=5032x1 Type=Byte, ColorInterp=Green
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 3 Block=5032x1 Type=Byte, ColorInterp=Blue
  Image Structure Metadata:
    COMPRESSION=JPEG

and there you have it: a spacially referenced jpg file.

GeoTiff - better performance

JPEG files are okay, but have poor performance when used in spacial environments. The best format is the GeoTIFF, so I used GDAL [gdal_translate] tool to convert it.

gdal_translate -b 1 -b 2 -b 3 -mask 4 -co TILED=YES -co COMPRESS=JPEG -co PHOTOMETRIC=YCBCR --config GDAL_TIFF_INTERNAL_MASK YES  cerilia-original.jpg cerilia-original.tif

For even more performances, overview images can be added within the GeoTIFF file using the gdaladdo utility.

gdaladdo -r average cerilia-original.tif 2 4 8 16 32 64 128

If we compare the end result, a MapServer "draw map" query takes around 680ms to return an image when using the jpg file and around 90ms when using the GeoTIFF file instead. This will make the map tiles rendering much faster.

Georeference additionnal rasters

One we have a georeferenced image covering the whole continent, its coordinates can be used as a base for additionnal smaller maps, which will allow them to correctly overlap on top of each other.

To do so, I used [QuantumGIS] 8. In the Plugins/Manage Plugins menu, make sure you have the Georeferencer GDAL one installed.

Open the software, then add the continent georeferenced GeoTIFF file as a new raster layer. Open the Georeferencer tool, the click on the "Open Raster" button and a small map (let's say a map of Roesone domain, for example). Follow the instructions, pick mercator coordinate system. You should see the map of Roesone now displayed in a window. Clicking on the map adds a point and a small window pops up where to input the coordinates. Instead, click the "From map canvas" button, which now shows the original Cerilia map. Click at the same location previously clicked and the coordinates will automatically fill themselves. I created a point for each province corners. Once you're done, click the "Start Georeferencing" button, follow the instructions. Your small map is now georeferenced using the same coordinate system as your original bigger map.

TileCache & seed

We seed each tile using TileCache, which then enables every files to be fetched directly as plain images instead of being rendered all the time.

Here's the seed command used to seed the tiles :

tilecache_seed cerilia_original 0 6 -b 266.667,-1680266.666,2683466.665,266.667
tilecache_seed roesone 0 6 -b 605772.827,-1508943.390,803826.446,1264488.637

References

  • [Map of Cerilia original JPEG image] 7
  • [World File wiki] 1
  • [World File calculator] 2
  • GDAL Utilities:
    • [gdalinfo] 3
    • [gdal_translate] 5
    • [gdaladdo] 6
  • [GeoTIFF raster format] 4
  • [QuantumGIS] 8
  • [TileCache] 9