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| Subject: | [gislist] Compressing Lines |
| Date: |
10/24/2005 03:35:01 PM |
| From: |
Bill Thoen |
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I'm trying compress all the lines in the US Census Bureau's TIGER data
set into as small a space as possible for use with a geocoder, so I'm
looking for ideas on how best to do this. I've managed about 60%
compression by using coordinates clipped to the nearest .00001 degrees
and applying a delta compression techique from there. (To do this I
store the first coordinate as a pair of signed long integers, then I
store the differece between those and the next coordinate as a pair of
bytes, then difference between the next point as a pair of bytes, and so
on. If the difference between subsequent points is greater than 255 in
either x or y, I just store the full coordinate and start again from
there.) I trim the coordinates to 5 decimals because for the purposes of
locating an address, the sixth digit contributes only a few feet to the
precision, and so it is not really necessary.
My question is, is there a better way to approach this problem to get
significantly greater compression, yet still retain good retrieval
performance?
- Bill Thoen
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