Re: [pct-l] The PCT GPS Mapping Project

At 11:15 AM 12/20/00 , Carl Siechert wrote:
>So is he/are you saying that the topo maps are off by that much?
>That would explain my GPS experience:
>But now you're saying the GPS is right and the map is not?

First off, **I** am making know claims. I don't know squat about 
cartography. I'm just repeating what Schaffer (PCT guidebook authero) told me.

Back when GPS first started to get cheap, I talked with Schaffer quite a 
while about it.

He said that his experience showed him that the USGS map data in remote 
areas was up to 1/2 mile off due to inadequate ground surveys to correct 
errors in areal photo data. He had discovered quite a few places where 
accurate lat/long positioning of the PCT treadway would disagree with it's 
position relative to features depicted on the map. He chose to draw the 
trail in the guidebook where the features said it should go, not where the 
lat/long said it should go. ("over the pass, not over the peak" was what he 
said.)

For all I know, Schaffer may have been off in his calculations of 
lat/long,though if the choice came down to Schaffer being wrong, or the 
USGS being wrong, I'm sure Schaffer would blame it on the USGS..<g>

OTOH, back when you did your test, there was an intentional error put into 
GPS signals to degrade it for non US Military users, so your 1/4 mile error 
may have been that instead. This error has since been removed.

-Brick

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