Creating Image Overlays for the Major Online Mapping Tools
Prepared for the Mapping workshop at Cleveland Public Library, March 19, 2008
Stephen Titchenal, Program Specialist for Technology, CH-UH City School District

Scanning:

Formats:
TIFF is the standard for archival digital images. The idea is to make a high resolution scan that can be used to create images in other formats for specific purposes. The original maps never need to be scanned again. LZW compression is lossless, if file size is an issue. Recovering a corrupted LZW image will be more difficult. For archival purposes, never use a lossy compression type.

Because low end scanners are not perfect and storage space can be an issue for large map images, you may decide to use a lossy file format. PDF and JPG are the two most common lossy formats today. I suggest scanning at a higher resolution to offset the loss of detail inherent in these formats. Each time you save an edited version in these file formats additional detail is lost. If you intend to edit these file formats multiple times you should temporarily save in tiff format. Always preserve the orignal scan as a backup.

Color/GrayScale/Bitonal(B&W)
I normally scan even grayscale/b&w maps and drawings in color. This makes it easier to remove stains, marks and blemishes while preserving the original inks. If an original b&w image does not need cleanup, then grayscale will save space and still allow threshold adjustments later.

Specialized display formats: DjVu, MrSID, JPEG2000, PDF
These formats can perform much higher lossy compression, while still preserving details. They require a separately installed viewer plug-in for display in a web browser. For the end users, djVu viewer seems to display much faster than the other formats. MrSID is often used for aerial images. JPEG2000 is used by ContentDM as the master image to render on the fly for display in a web browser without need for a separate viewer.

Backups:
1. Burn multiple DVDs of original and edited masters.
2. Copy to multiple hard drives. External USB hard drives are very inexpensive.
3. Re-copy files every few years - comparing file size after copying.

Image Enhancements (Gimp - http://www.gimp.org/)

• View->Zoom (change image view size as appropriate for editing task)
• Image->Scale Image->(use Width/Height Percent or Points to resize image)
• Straightening Image:
  • Tools->Measure - displays angle of measurement along map line that should be horizontal
  • Tools->Transform->Rotate - arbitrary - enter angle determined above.
  • Image->Fit canvas to layers (forces canvas to fit rotated image)
• Cleaning up Image - Optional (always clean up copy of original image)
  • Edit->Undo (remember you can restore image if you are not happy with changes)
  • Colors->Auto->Normalize or Colors->Levels (to adjust manually)
  • Colors->Maps->Color Exchange (remove color blemishes - paper browning, etc.)
  • Selection area->delete key to clear selected area (clean up sections of image)
  • Colors->Threshold (removes all colors/grays - useful for reducing image size)
• File->Save As-> (Choose file format needed - jpg, png, etc.)

Yahoo: MapMixer tips (http://maps.yahoo.com/mapmixer)
Advanatages: free, easy to use, stores maps on Yahoo's server.
Disadvantage: Cannot edit once saved. Can delete and recreate. Limited file size.
Edit maps to a resolution under 4000 x 4000 pixels before rendering. I have found that maps close to this size may still freeze the rendering software.

Microsoft Virtual Earth: Map Cruncher tips (http://research.microsoft.com/mapcruncher/)
Advantages: free, easy to use, generates multiple zoom levels to speed display for users.
Disadvantages: multiple zoom levels can increase total size and rendering times. Must have space on a file/web server to share over network.
The help file that comes with map cruncher is very well done. These tips highlight choices that are easy to overlook the first time you create a map set.

1. The higher resolution the orginal map image is, the sharper the map will be at higher zoom levels. But the time to render and the number of tiles will increase exponentially. When rendering choose a zoom level that complements the resolution of the map. Zoom level 19 corresponds to street level. Only zoom to that level if your map was drawn and scanned with that level of detail.

2. A layer is a single tile set that is always viewed at once. It may comprise a single source map, or multiple source maps. If you want to be able to turn individual maps on and off put them in separate layers.

Google Earth/Maps tips (http://earth.google.com/)
Advantages: Most interactive mapping tool. Offers many advanced options with KML.
Disadvantages: Higher learning curve for some features. Rural areas do not have as high a resolution satelite images as Microsoft Virtual Earth.