[gdal-dev] Compact bit representations of geometry for AIS binary messages?

[schwehr at ccom.unh.edu]

Hi All,

Sorry this is off topic, but I figured this is probably a good place
to ask this question as you all work with a wide range of systems.  If
you want to respond off list or comment on
http://goatbar.livejournal.com/314.html , that would probably be the
best.

I am looking to propose a couple a binary messages for maritime
communication that must specify regions in a very limited message
size.  I'd like to be able to handle a range of different geometries
using a couple different messages to obtain the most geometry being
sent with the minimum amount of bandwidth.  Something standard like
WKB would be nice, but uses too much space.  An example of the
existing message to define a point is the lat/lon fields in this
message:

http://vislab-ccom.unh.edu/~schwehr/software/noaadata/ais-messages/ais_msg_3.html
http://vislab-ccom.unh.edu/~schwehr/software/noaadata/ais-messages/structs-inc.xml

Where a point takes 55 bits.

I figure a circle is reasonable easy by just adding a radius.  For a
"rectangle", I'm wondering if a point with a dx and dy (in meters or
1/600000's of a degree) would be workable rather than two corners.

Then there comes the necessity of specifying an arbitrary polygon.  I'm
thinking that I have to bite the bullet and come up with a max number of
points two fit within the bit constraint of the max message size that I am
going to allow (368 bits).

Any thoughts?  I'm trying to propose a standard that doesn't do more harm
than good :)

Thanks much,
-kurt