Ground Elevation Question

I noticed that my Stratologger CF reports negative values for ground elevation and I'm not sure how to interpret that information.

Example: Google Earth show the elevation of my launch site is 110 ft. So I launch a rocket there and look at the SLCF's recorded flight data. It shows:

Apogee: 2,832 ft. AGL
Ground Elevation: -191 MSL

If Google Earth shows my launch site elevation as 110 ft how do I rationalize a SLCF Ground Elevation value of -191 ft?

The altimeter only has barometric pressure to go on, and pressure changes not only with altitude, but also with current atmospheric conditions (temperature, pressure fronts, etc). So local pressure was probably a bit higher than standard for your launch site altitude.

That makes me curious to know under what reference parameters I might see the "Ground Elevation" reading align with the what I believe the ground level to be - 110' ?

Altimeters are based on the International Standard Atmosphere. A handy calculator is here: https://www.digitaldutch.com/atmoscalc/

"Normal" conditions at 110' above MSL are 58.6 degrees F and 29.80 in of Mercury. (The pressure is the important part for the baro sensor). Any deviation from that will result in a different altitude reading.

Pilots need to correct for this all the time, using the atmospheric pressure from the weather report (which is always reported at sea level) to calibrate their altimeters.

Again, thank you. I will review the information at that link, and attempt the correction at our next launch (I have a Davis weather station that reports barometric pressure to the nearest 1/100th of an in. Hg )

Here are 2 different explanations you might find useful, along with 2 wiki references.

https://www.langleyflyingschool.com/Pages/Flight+Operations--Pressure+and+Density+Altitude.html

https://www.aopa.org/asf/publications/inst_reports2.cfm?article=2910

https://en.wikipedia.org/wiki/Pressure_altitude

https://en.wikipedia.org/wiki/Density_altitude

Bob

The bottom line, the Ground Elevation MSL is the difference from the standard atmosphere for sea level. The altitude is still the feet above the starting point using the current sensed conditions. That flight would still report 2,832 ft even if the Ground Elevation was +427 ft., assuming the rocket flew to the same physical height in the lower pressure atmosphere.