Current data from airport 15 miles away at elevation 1001 feet above sea level reports 21F and 30.45 altimeter.
I would set the EDM for 25F as I'm starting now and the temp will raise about 10 degrees befoe I'm done.
Using the magic one inch per thousand feet correction, I should use 29.45 for the pressure setting as I'm at the same elevation. However, I normally leave it set on 29.0.
Longest shots today will be 1000 feet.
How much is the maximum error I'm creating by not setting my corrections more precisely or more frequently?
> Current data from airport 15 miles away at elevation 1001 feet above sea level reports 21F and 30.45 altimeter.
>
> Using the magic one inch per thousand feet correction, I should use 29.45 for the pressure setting as I'm at the same elevation.
If the pressure at the airport is 30.45, and you're operating at the same elevation as the airport, wouldn't you want to set your pressure at 30.45?
They are indicating a barometric pressure corrected to sea level of 30.45. They are actually 1001 feet higher than that for a correction of about 1.0.
Current numbers are 30.49 and 28F.
[sarcasm]So that other post made you feel like a slacker too?[/sarcasm]
When you are set up on that 1000 foot shot, try an experiment. Change the temp up and down 20 or 30 degrees. Then do the same with pressure. It will depend on equipment and location, but I have found that the temp makes virtually no change in 1000 feet. The pressure does make some difference, but I have experimented with this enough that I only need to check my settings once a month to stay within .02 in 1000 feet.
If you try this, please let us know the results.
Scott
Jim, you're joking...right?
Current conditions at my house.
Barometric Pressure = 23.44 in.Hg (mean of two Thommem barometers)
Temperature = 32° F. (Mean of two Taylor Thermometers).
GTS_3b +49ppm
National Weather Service (@ airport, several miles away, SAME elevation).
Barometric Pressure = 30.36 in.Hg
Temperature = 30° F.
GTS_3b -20ppm
Difference = 69ppm
1,000 foot shot = error of 0.07 feet.
2,640 foot shot = error of 0.18 feet.
BTW, it's over 1,000 vertical feet from my front door, to Main Street (the saloon) in Town (about a mile away). I have had projects with OVER 6000 feet of relief, and 2-3k feet is about the average in these parts. Hell, Sections with 2000 feet of relief are not all that rare.
As always, IT DEPENDS.
Loyal
> Jim, you're joking...right?
Not joking, but then I work at about 50' MSL. I guess I'd heard the corrected-to-sea-level thing before and just forgotten about it.
No worries Jim
Heck, I just subtracted 6750 from 7100 and got 1000 !!!
I must be getting senile(r)
🙂
Loyal
Never found it a hardship to keep a thermometer and barometer in the EDM case, now in the Total Station case and then reading them as part of the setup routine or reading again several times during very long occupation. Today some instruments have that capability to produce their own corrections but I question if it is a reliable method in using that capability, unless you always setup in the shade and give the gun 30 minutes or so to acclimate. Short distances allow for some leeway in ppm corrections and few today use the EDM for obtaining long distances, if you do, own a thermometer and a field barometer which you have taken to the airport and adjusted. Establishing the habit of reading those instruments takes little time in the field.
jud
