60cs Altitude Calibration

[+mtedelen]

When I try to calibrate the altimeter on my 60CS it asks if I know my current barometric pressure at my location, but i am thinking that they mean sea level? When I try to put in the correct pressure for my area (reno) it will not even let me go that low. This makes me think that they mean sea level pressure. Any ideas?

[Old-Time Wanderers]

No, the calibration is at your location. Go to www.weather.com and get your local weather by typing in your zip code. Your local weather will provide the barometric pressure for your general location. Unless you are in the mountains, the pressure should be pretty close to what it is at your location. If you are using inches of mercury, each 1/100th of an inch of mercury equals about 10 feet in elevation for example, at an ambient pressure of 29.92 at sea level, your pressure will be about 29.90 if you are 20 feet above sea level. Your autocalibrate by gps should come within about 9 feet (3 meters) of your actual elevation.

 

Just use the barometric pressure on www.weather.com and you'll be in the ball park.

[peter]

No, the calibration is at your location. Go to www.weather.com and get your local weather by typing in your zip code. Your local weather will provide the barometric pressure for your general location.

www.weather.com and similar weather reporting sources will not give you the actual atmospheric pressure at your location, but rather the adjusted pressure: what it would be if you were at sea level. For example, the www.weather.com site currently shows the pressure at Tahoe City, Cal. to be 30.11" Hg. But Tahoe City is at about 6300' and the actual atmospheric pressure there is more like 24" Hg. The reason the weather reports use an adjusted pressure is to make it easier to compare readings from various locations and relate them to weather conditions. So an adjusted pressure of over 30" is relatively high and usually means good weather even though the actual pressure at the site up in the mountains may only be 24"Hg.

 

I think mtedelen is correct and the 60cs is expecting an input of the adjusted atmospheric pressure, i.e. what it would be at sealevel. However, in most cases I'd suggest letting the unit auto-calibrate from the time-averaged GPS readings, especially if you can give it a good skyview.

[+mtedelen]

Thats what I thought. From what I know most weather sites are reporting BP at sea level. I did not realize that my unit auto calibrates itself.

[Old-Time Wanderers]

I'm sorry. I see that I wasn't clear at all in my response. I didn't mean that the pressure was correct at your altitude. Just that it was correct at your location (at sea level). That's why I provided the conversion factors. Pressure decreases be about 1 inch of mercury (1.0"Hg) for each 1,000 feet that you increase in altitude. It loses goes down by about 0.1"Hg per 100 feet and 0.01"Hg for each 10 feet. If you are near Reno/stead airport (FAA identifier 4SD) your elevation is about 4,214 feet. If you are near Reno/Tahoe International (FAA identifier RNO) then your elevation is about 5,050 feet. Let's say you're near Reno/stead. Your pressure from weather.com is 30.00"Hg. The pressure you would key into your GPSr would be 25.79 (reduced 4.21) for your altitude of 4,214. If you are near Reno/Tahoe Int., you would input 24.95 (reduced 5.05) for your altitude of 5,050 feet. While it's not exact, it gets you into the ballpark.

 

Again, I'm sorry if I confused. It's usually better to just use the "calibrate by GPS" selection. Just select "no" for the questions of "do you know the correct elevation" or "Do you know the correct pressure." Another way is to simply pick up a topographic map of your area, locate your position and key that in as the "correct altitude."

[+CompuCash]

I'm not sure I understand. The manual does not say anyting about corrrect pressure for sea level. The manual says "Pressure at your location". I put in the pressure here and it says 130' when I am done. This is fairly corrrect. I think it is 150' or so here. I could set it at 0 when by the Bay (same as ocean) and check it but that is about close enough.

 

Sugget you call the Reno Airport and get th current pressure and use that.

 

cc

[+WxGuesser]

use this link and get your current pressure.

 

http://www.uswx.com/us/wx/NV/KRNO/

[[MTB]_Intrepid]

If I'm out in the woods and forgot to set my altimeter, I use the GPS altitude (always available when the little 3D symbol appears) and enter that to give a "ballpark" value to the altitude plot. Yeah, it's off by 20 or 30 feet sometimes, but it's better than nothing. On the calibration screen just say "no" when it asks if you know the pressure or altitude, then it will ask if you want to use the GPS altitude. You must have at least 4 satellites in view for GPS altitude to be calculated.

[[MTB]_Intrepid]

Drat. Double post removed. CGI meltdown...

Edited March 28, 2005 by [MTB]_Intrepid

[Old-Time Wanderers]

I just recalibrated my barometer using the pressure given by www.weather.com. When I turned off the " Use with GPS" and inputted the pressure I "adjusted" for my actual altitude, I got a reading that was way too low. When I inputted the "given" pressure at sea level, my altitude was correct. I must conclude from that test that the GPSr takes the "actual" pressure at "actual" altitude, and subtracts it from the sea level pressure. It then determines the actual altitude from that result.

 

Bottom line is get the sea level pressure from any of the above sources, and input it as your pressure at your location. The GPSr will do the rest automatically.

[+CompuCash]

I just recalibrated my barometer using the pressure given by www.weather.com. When I turned off the " Use with GPS" and inputted the pressure I "adjusted" for my actual altitude, I got a reading that was way too low. When I inputted the "given" pressure at sea level, my altitude was correct. I must conclude from that test that the GPSr takes the "actual" pressure at "actual" altitude, and subtracts it from the sea level pressure. It then determines the actual altitude from that result.

 

Bottom line is get the sea level pressure from any of the above sources, and input it as your pressure at your location. The GPSr will do the rest automatically.

 

thanks - that's what I said above - the compensation just did not make sense.

 

cc\