[GeoStL] Real Geeky techno stuff...

-
 From a cacher in the PNW.  It is way over my head but looks like it might
beinteresting for geo-fiddlers out there...   

The new cache queue bas been very full. Looks like lots of folks are getting
out this W/E. 

glenn



I consider averaging mostly a waste of time. If you're not averaging 
continuously over many hours or even days, you're probably just 
averaging a few good fixes with a lot of mediocre ones and a few bad 
ones. Better to take just a few good ones. 

How do you predict good ones?  Download the free satellite prediction 
software and almanac from Leica Geosystems: 
http://www.[1]leica-geosystems.com/gps/almanac/index.htm   The program is
really  pretty straightforward and easy to use. There are other similar 
programs out there, but I'm used to the Leica one. If there are trees 
around (sounds like it from your trouble), set a mask angle of at 
least 15 degreees and pick the time of minimum GDOP to take your 
fixes. GDOP dips below 2.0 several times per day (with a 15 degree 
mask). The best time today in the Puget sound area was just after 
10am, but that might not have been the best time at your location 
because trees might have blocked one of the satellites needed for that 
good GDOP. In that case you can tell because your GPSr won't report a 
very good EPE. What you should do then is stick around till the time 
of maximum satellite availability (that was right around noon today) 
to give your machine maximum choice. If you still can't get good 
fixes, you can just live with it and add a star to the difficulty 
rating of the cache. Another thing you can do if there is clear space 
nearby is back off where you can get a good fix, take a careful 
bearing and distance to the cache and calculate cache coordinates from 
the good fix.

I know this must sound like a whole lot of new gobbledegook to
absorb, but I'll answer any questions you have as best I can. It 
doesn't really take a very deep understanding of the system to use it. 
I don't bother with too thorough an understanding myself, just barely 
enough to get the results I want. I don't do any math or anything like 
that.

Note: If you took your first readings late (after 10) Sunday morning. 
they were probably pretty good ones. If you then returned with 
daughters after 4pm then you were in the worst time of the day, when 
the few satellites available were close to the horizon, making for 
large atmospheric errors, blocked satellites etc. These days there is 
a big hole between 4pm and 7pm when all the satellites are very low
on the horizon and a GPSr is almost useless on the ground in much of 
Pugetopolis. That hole gets a few minutes earlier each day so that it 
drifts about two hours a month. 
---------------- Glenn 

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