David,
Problem is not app related but GPS related. Altitude is not measured as accurately with civilian gear. Another option is barometric altimeter.
Beg, borrow or buy one. Perhaps, your local Airplane Owner & Pilots Association can provide one removed from an aircraft, or you could get one from REI.
A lot depends on what accuracy you need and over what time frame. Over a period with steady barometric pressure, you could do fine, but if a storm is approaching or leaving, the barometric pressure may change over a short time period that could make your readings less useful.
I would check with an avionics repair facility to see if they have something that would work for you.
For what you want to do, one cheap approach may be to use the pipe and add a transparent section of hose with measured markings.
Attach to pipe and hold vertically on the downhill end. Fill the pipe with water on the uphill side. The height of water in the downhill side is the same elevation as the top of the water in the uphill side. Read the height in the marked transparent tube to know the elevation difference,
Paul
This post doesn't help David with his pipe, but rather tries to clear up some GPS misconceptions about accuracy.
Paul, when you state "civilian" GPS gear is not as accurate as aircraft or even military GPS units, that is still somewhat true, but, not like it was when the GPS constellation went on line and through the 90s.
Yes, DoD did at one time restrict reception of algorithms within the DoD second frequency to reduce accuracy. This was due to a finding such was a "national security" interest. US officials were afraid adversaries would use the accurate signal against us. This program was called "Selective Availability". At the time aircraft/pilots wishing to fly to very low minimums using GPS had to install a special receiver to receive a GPS signal for the accuracy input from the restricted, "selective" broadcast by a low power directional transmitter near the runway. Today, such is not needed or used since "Selective Availability" is a thing of the past.
The accuracy issue today is if your GPS receiver (cell phone, Garmin unit, etc) is capable of receiving BOTH frequencies transmitted from each satellite within the constellation. If so, you will have extremely accurate location indication, assuming of course all other variables for achieving such are not compromised.
Many handheld Garmin units, rather than receiving both frequencies from the GPS constellation, receive one freq from GPS and one freq from GLONASS (Russian constellation). This helps keep the unit reasonably "handheld" and achieves greater accuracy. Interestingly, due to satellite positioning, GLONASS will provide better accuracy than the GPS constellation. If you have the ability to enable both on your unit, always best to do so. I know of no consumer cell phone that receives both frequencies from GPS or one from GPS and one from GLONASS.
But one should never ever follow a GPS unit regardless of the device without paper map available for reference. I still use paper maps to backup my GPS unit. On foot, I will shoot a long range GPS bearing to a known feature and verify with map and compass before moving out. I then use my map and orienteer to the destination. And a straight line course is very rarely the fastest route. But I'm old school. I was taught orienteering and celestial navigation before Armstrong set foot on the moon. Going paperless might be a good thing for the most part in today's society, but, never in the back country!
Bottom line, if you need accurate GPS signals for back country navigation, use a quality unit designed for such and not a cell phone. And backup with and use a paper map!
Info from Uncle Sam.
Edited to correct typo Steve quoted. 
Edited by Advmoto18, 11 February 2020 - 01:26 AM.
