In reply to K6DGW:
Hi Skip and all,
It’s a very good idea to compare your GPS readings with an accurately determined position and/or height. Do it frequently and record the readings.
This is a long post and I apologize for that. However GPS and barometer accuracy discussions could fill a book. The discussion of different globe shapes and what is best is another book.
Consumer GPS devices are less accurate than we would like to think. We have been seduced by digital displays and apparently stable readings.
Professional GPS surveying instruments use different signals from the GPS satellites cf consumer ones and if you pay enough money you can get one that will give a reading with an uncertainty of a few mm if left to do the number crunching for a day or more.
The resolution on the typical consumer GPS display, 1 m, should not be confused with the real resolution of the measurement. Any kind of resolution should not be confused with accuracy, now referred to as measurement uncertainty.
The internal number crunching has limited resolution, usually worse than a metre. More resolution costs more electricity and dollars, and using different satellite data.
The measured results calculated in the GPS typically vary randomly by a metre or more from one measurement to the next and readings often follow a rough cyclic pattern of maybe 10 m. You can reduce the random errors significantly by averaging the readings over say half an hour - if you have half an hour. However averaging over only 5 minutes helps.
The systematic errors are dependent on your device, the satellite constellation and the ionosphere. And of course there are temperature, altitude, battery voltage and local signal reflection effects to consider.
Oh and the Circle of Probability represents a 68% probability the true value lies within that circle. Double the COP to get a 95% probability. Multiply the horizontal error by at least 1.5 for the vertical error.
Cheap hand held altimeters are not necessarily better and their “accuracy” might be a 68% probability value, or it might have come from the Sales Dept. Use them only for relative measurements unless independently calibrated more than once.
I use both a GPS and a small altimeter both of which have claimed accuracies of 5 m vertical. I measure the AZ by walking down from the trig point or cairn until both indicate 30 m change. This is the most optimistic determination I can make. When I have the opportunity I recheck these measurements usually by immediately walking back to the peak.
Can you use spot heights or paper maps to check your GPS? Yes, but … Geodetic reference peaks will certainly be determined to within 1 m of the defined sea level, other peaks are less well determined. Official Australian government survey paper maps generally have a contour line accuracy of 70% of the distance between the contour lines and spot heights may be in error by up to 3 m to 5 m. I understand this meets international standards. Contour lines are often only at 20 m intervals except in some limited areas where 10 m or even 5 m might be available. Hence there is still plenty of work for surveyors to accurately determine local terrain for a variety of reasons.
If you use an instrument carefully they are a wonderful asset but if you rely on them without accounting for some uncertainty then you may be fooling yourself. As a rule of thumb I would take a reading of 30 m below the summit as maybe beyond the extent of the AZ and a reading of 20 m as being maybe within the AZ but I would really want to do a difference measurement between the known summit (cairn or similar marker) and the operating position.
Differential measurements always help reduce the systematic (offset) errors.
Good luck and stay on the tracks.