ridecamp@endurance.net: GPS measurement

GPS measurement

Jim Mitchell (navion@lsbsdi2.lightspeed.net)
Sat, 08 Feb 1997 19:36:34 -0800

Since I got several questions on my GPS unit I thought I would answer them here.

The older GPS units (which I have one also) only gave the mileage from point to point
and even though you could have routes with 20 points or so it still didn't give accurate
trail mileages. The new units (like within the last year or so) now have an odometer
function that constantly sums the distances your traveling and keeps that sum. I heard
one unit records a point every 4 seconds and then straight lines between those points
for the odometer. That would be good enough with minimal error. However my unit says it
updates every 1 second or continously in the standard mode so now even that error seems
to have been reduced. I rode with it today with the antenna suction cupped to my helmut
and it worked great. Again there are no trees where I rode.
I would be worried about its accuracy in the forest, this GPS tracks 8 satelites but
only needs three for 2D accuracy. It says it will continue functioning even if trees or
whatever block some satelites as long as 3 are still in view. I will try it in our
forests at some point and give you some feedback. Obviously it would depend on how dense
the forest is.
I got all kinds of comments on my look with the antenna sprouting from the top of my
helmut. That might be a bother to some people. The cord ran from the antenna down to the
small unit which I carried in the front pocket of my wind shell. This unit uses 4 AA
batteries and says they will last 12 to 20 hours. It has a fuel gauge graph that tells
the condition of the batteries. Judging by my ride today of 4.5 hours I would say they
will last about 10 to 12 hours but I don't know yet.
This unit stores your route in it's memory and can sort of draw it on the moving map.
It saves up to 768 points for your route. It picks the points based on direction changes
or at set time intervals at your choice. Today I left it on automatic and it used 725
points in our 19.5 mile ride. This would be a point about every 150 feet. The points
would be closer on the winding parts and farther apart on the straighter parts. You
could set it on time and get points at whatever interval you wanted. You can buy the
download/upload software and cord for this unit and download data to your PC and then
print it out to any scale. This would enable you to overlay a USGS map and get accurate
trail positions. I'm not sure how many points it will download but I would hope all 768
of them. But this is not how it does the odometer so the odometer is more accurate than
I bought mine at WestMarine which is a boating chain and they have a catalog and mail
order. I don't have their 800 # but their local # is 805-654-8233.

The costs follow:
Garmin GPS45XL List around $300.00 Reg discount $250.00 on sale now $199.99
Suction cup cord to detach and remote antenna $23.99
Soft foam case to help impact damage $11.95
PC software & cable $89.99

I didn't buy the PC kit but probably will have to now.
I'll try it at 20 Mule team so it will be interesting.

Jim Mitchell
Bakersfield, Ca.

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