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Crater Lake National Park: Flattening Overlaid Images
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What: Have you noticed that the drawing time of a map increases with the number of raster layers used to symbolize it? This can be alleviated pretty easily. Why? It can get tedious waiting for your map to redraw. This is not always necessary -- here we describe how you can decrease the number of raster layers used on your map with no loss of visual quality. For the Crater Lake map, once we had elevation, bathymetry and vegetation (bump map) symbolized, we quickly got tired of how slowly it was drawing. Three raster layers were used to draw the elevation, three more were used to display the bathymetry and two were used to draw the vegetation. Even though we had trimmed the layers of the map down to the bare minimum, it was taking ten or more seconds to draw. By flattening out the raster layers, we were able to get the map to draw in a little under four seconds with no loss of cartographic quality. How: By flattening, we mean combining the content in these raster layers into one image, making it possible to draw the background of the map with one layer instead of what was ultimately nine layers (we also decided to flatten the buffered park boundary).

This effect works best for raster layers with no text -- if you flatten vector layers or text the linework will become pixellated, especially if you use a coarser resolution. Try this yourself with and without vectors and text to see the differences for your map.

Here's how to flatten the background raster layers:

  1. In Layout View, zoom to the extent you will be using when you print or publish your map.
  2. Switch to Data View; this effect will not work in Layout View.
  3. Click the Fixed Zoom Out tool once.
  4. Change the size of the ArcMap window so your data view mimic's the shape the data frame occupies on the layout. This is important as we don't want to save extra data when we flatten our map's background layers. You cannot do this effect with your ArcMap window maximized.
  5. Turn visibility off for all the layers that will not be flattened. This includes any annotation groups as well.
  6. Export the map to JPEG format. Start with 300 DPI, set the JPEG Quality slider to Max, and turn on the option to Write World File.
  7. Check your results. Turn on only the layers you flattened. Then add the JPEG file to your map and zoom in to an area. To see if you can tell a difference, turn the JPEG layer on and off and compare the results to the original rasters. You may need to export the map several times with different DPI and quality setting to arrive at the best resolution. We used 600 DPI when we flattened this map's background images. If the JPEG looks good on the first try, try using a smaller resolution, like 180 DPI and see if you can produce a smaller file as that will draw even faster.
  8. Remove the flattened layers from your map and use the JPEG you created instead. Turn on your other layers and enjoy your map's faster drawing performance.

An added benefit of flattening so many images was that we were able to decrease the size of the images in the data download for this map from over 120Mb to 6Mb (without pyramids, 18Mb with pyramids).

Great way to increase the display speed. posted by Prashant Hedao on Jun 26 2007 9:11AM
It is really good to know the Flattening technique. This not only substantially increases the display speed but also reduces the data size.
Exactly! posted by Aileen Buckley on Sep 14 2010 2:37PM
Exactly right! A great space and time saver all around!
Colors not right posted by Matt Stull on Feb 18 2014 9:52AM
The colors on my output JPEG image don't match the original raster layers. I'm not sure what I'm doing wrong but I have some ideas. I'm using 3 raster layers that make a Swiss Hillshade effect. Each of the three layers uses a 55% transparency and they are all set to Bilnear Interpolation for their Resampling method. Any ideas?
Check the stretch on the symbology tab posted by Aileen Buckley on Feb 18 2014 11:00AM
On the Layer Properties dialog, on the Symbology tab, check that there is no stretch being applied -- I think the Stretch Type defaults to Percent Clip and you want it to be None.
More Feedback posted by Matt Stull on Feb 20 2014 1:43PM
If I set my stretch to "None" then my color ramp gets all messed up and everything is one color. Are you saying to not use the "Stretched" choice on the left side of the Symbology window in the Layer Properties? Thank you for responding!
Stretch posted by Aileen Buckley on Feb 20 2014 4:42PM
Usually there is not a color ramp per se when you display the JPEG. If you use the Stretched renderer (on the left side of the Symbology tab), then set the Stretch Type to None (at the bottom of the Symbology tab).
That worked! posted by Matt Stull on Feb 21 2014 12:21PM
Thanks for the tip, now the JPEG looks good!
Great! posted by Aileen Buckley on Feb 21 2014 1:48PM
Glad you asked so others know this now, too!

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