# What is the local accuracy of the Garmin Oregon 750 barometric altimeter?

I want to manually create a topographical heightmap of a hillside property with the highest possible accuracy for the altitude. This will later serve as the basis for planning terraces and building placement. The only piece of equipment that is available to me is my old Garmin Oregon 750t. The property is roughly 2400 m² and has a lot of landmarks (small paths, brushes, some rocks) and border stones which I can recognize in aerial photos, so longitudal and latitudal accuracy are not the issue here.

My concern is that the accuracy of the Garmin's built-in altimeter says ±15 meter (or ±50 feet) Source.

For my purpose the accuracy of the absolute altitude would not matter, but the relative altitude on the property should be correct with roughly ±1 m deviation (regarding to an arbitrary chosen reference point, e.g. the south-eastern border stone).

So assuming the following for the measurement of multiple location on the property:

• Correctly calibrated altimeter
• Mostly constant local temperature
• Mostly constant local air pressure

I guess it will take me about an hour to walk over all points and write down the data, so the above assumption ought to be correct (hopefully).

What is the (estimated) local accuracy of these altitude measurements?

• If you can save your walk as a track, you may be able to download and look at the individual points without having to write anything down. And at a high sampling rate you just walk back and forth and you can do some spatial averaging to converge on a 3D map of the property. – Jon Custer Aug 4 '20 at 19:15
• Depending on the needs for accuracy, you will likely get better results using a carpenter's level and measuring tape. That is taking into consideration the very low resolution of the Garmin. – wallyk Aug 4 '20 at 23:36
• Personally I would get a slope meter, a tape measure, and get familiar with trigonometry... You only want the relative heights so the altimeter, while it still has inherent error of +/- 15m, that error is likely to be the same for each point (at a guess), you should be fine. Worst comes to worst - just do multiple measurements at each point on subsequent days, work out an average difference between them all. – bob1 Aug 5 '20 at 0:25

## 2 Answers

Averaging would help a lot. I assume the precision is smaller than the accuracy, as it is on my old etrex and all phones, with or without barometers (it probably reads to the nearest metre, maybe even the nearest foot).

Ideally you'd survey exactly the same spots multiple times, and for each spot work out an average height. I would offset (subtract a constant value from) each set of heights so the your datum (SE border stone) is at zero, then average, then add back the best value you can get for this point at the very end. Offsetting like this avoids issues from calibration differences.

The difficulty is that you need about a factor of 10 improvement. Assuming random error, this would mean 100 surveys. However I suspect that the stated accuracy is worse than a best case, so by surveying on days when the weather is constant and holding the device consistently you'll already do better than 15m, and could reduce the number of surveys. You can test by leaving it logging in one place for the length of a typical survey (your datum stone, then you could use this data) and looking at the standard deviation of the resulting log. Logging to a GPX file would allow easy extraction of the heights, perhaps using GPSbabel to convert to a CSV file.

Use a marked stick or pole and a sighting level. I got the level at a building supply or do it yourself store. You mark the stick in say 0.1m increments. One person stays in a same spot and another goes to different locations. You look through the sighting level and with its bubble you level the sight and read the mark on the stick and record them on your map. With a reasonable sized you'll have to move down the slope several times.