OnYerLeft
new member
Reged: 06/07/04
Posts: 35
Loc: CA, USA
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i just got a computer with an altimeter. the manual states that i have to set the altimeter function based on some topography values or numbers. is there a website where i can obtain topography numbers on specific areas? if not where else can i get this info? thx.
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flythebike
captain
Reged: 08/26/04
Posts: 272
Loc: N. Virginia, USA
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FAQ about Topographic maps from the US Geological Service/Survey: http://interactive2.er.usgs.gov/faq/list_faq_by_category/get_questions_for_category.asp?category_id=26
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vaxn8r
contributor
Reged: 12/19/03
Posts: 222
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Basically you just need to know the altitude of where you start your rides. Then you need to reset every ride because atmoshperic pressure variations will cause your alt reading to wander from day to day.
One trick is to go to the airport or a known elevation, set your computer and drive/ride home and get your home reading. Then you're ready to go. Dont bother with topo maps because if you aren't at the top of a mountain they aren't going to give you accurate enough data...unless your house happens to be on the map I guess.
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OnYerLeft
new member
Reged: 06/07/04
Posts: 35
Loc: CA, USA
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thx for the info. i'll check usgs' website. vaxn8r's tip sounds practical though. good ridings! 
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Siriusguy
new member
Reged: 03/28/04
Posts: 11
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Don't take the altimeter of a bike computer too seriously. My Polar S710i somewhat under reads altitude differentials. As far as the "ascent" figure or total cumulative climb, I found that the unit recorded over a recent perfectly flat ride circling a small lake an "ascent" over one hour of about 60 feet. For my short training loop of about an hour, the "ascent" figure has varied from 265 to 350 feet. So, the relative accuracy of such a unit really is best for a ride that has a moderate to major amount of climb, and even then, expect some uncertainty. It certainly is interesting to start out a ride from my house at the know altitude of 375 feet, then return home finding that due to barometric shifts after several hours, my garage has been relocated up or down at least 100 feet!
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