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Get Barometric Pressure from Your GPS

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/16/2010 at 07:20
jonoMT View Drop Down
Optics Master Extraordinaire
Optics Master Extraordinaire

Joined: November/13/2008
Location: Montana
Status: Offline
Points: 4810
By itself or combined with altitude, barometric pressure is one of the two most important atmospheric variables that will effect your bullet's trajectory. (The other is temperature).

There are different ways to get barometric pressure information, such as using a handheld wind meter that has a pressure sensor or by checking sites like for the pressure reported at the nearest local airport. However, if you don't own a wind meter or don't want to carry yet another device in the field, but have a GPS unit such as the Garmin 60CSx, there is an alternative. Using the pressure reported at the airport isn't always useful either. Even if the airport is near by it may be 2000 feet lower and face completely different weather conditions.

GPS units like the 60CSx have a built-in pressure sensor, which is included to help track elevation gain or loss. If you calibrate it at a known elevation, it will provide reliable, current readings. And you can choose between either station pressure or corrected pressure. (Using station pressure, you enter that value in your ballistics program along with an altitude of 0. Using corrected pressure, you must also enter the current altitude, which you can, of course, get from your GPS).

Before calibrating, since I am near a local airport and am only 300 feet higher, I do like to check the 3-day history, which reports not only station/corrected pressure, but also temperature and humidity:

1) Choose the units you are accustomed to using with your ballistics calculator, e.g. inches of mercury (hg). To do this, go to the Main Menu, select Setup, then select Units and scroll down to Pressure. Select the units, then hit Quit:

2) Next, use the Page button to navigate to the Altimeter page:

3) Then hit the Menu button and select Calibrate Altimeter

4) You will be asked if you know your correct elevation. If so, select Yes. If not, you can either enter the current pressure (if you are close to an airport) or choose to use the current elevation if you're reasonably sure the GPS is reporting it correctly.

5) Use the keypad to enter the correct elevation, then hit OK when you are done.

6) Hit Enter to go back to the Altimeter page. You should then see the pressure reported. On my unit, this defaults to the uncorrected station pressure (what Garmin terms "Ambient Pressure"):

7) You can switch to corrected pressure by hitting Menu and selecting Plot Barometer:

8) To switch back to station pressure, hit Menu and select Plot Ambient Pressure:

Remember, if you choose to use corrected pressures, you must enter the altitude in your ballistics program or you will end up with a much greater drop if you are significantly higher than sea level.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/19/2010 at 23:02
helo18 View Drop Down
Optics Jedi Knight
Optics Jedi Knight

Joined: December/02/2006
Location: Montana
Status: Offline
Points: 5550
I have the 60CSx and have used the Barometer a lot.  And I am only about a 100 ft higher than the airport here it is easy to calibrate and use.  I have used it for so many things.  I also have the airport ASOS numbers in my phone from flying so I can call the airport and get the weather at any time for a lot of the state.  Makes it nice to calibrate and check the GPS if I am somewhere with no internet.
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