GPS positional accuracy III


Hi all,

this is a short follow-up on my previous tests (I, II) with u-blox different u-blox M8 GPS modules.
This time I'll report about a survey campaign of several days with different copters and GPS. After processing the data I was surprised about the positional accuracy so I decided to share some numbers.

We used two copters running AC 3.3. One with an M8N and the other with an M8T GPS. The survey was conducted over a period of 4 days.

Six orthomosaics were overlapping:

Dataset   Day time
A         3   10:40
B         1   11:20
C         4   12:00
D         4   10:20
E         4   11:00
F         1   10:00

The orthomosaics were generated using direct georeferencing, i.e. without any RTK processing or ground control points. Hence, the final accuracy, or to be more precise the spatial offset between the maps, is a function of the accuracy of a single GPS and the image processing pipeline.

Since the flights were conducted over 4 days with different GPS and copter setups, the positional offset shows the overall GPS accuracy that can be achieved with M8 GPSs. The processing chain was the same for all datasets. However, there is for sure some influence as well. [I'll provide more details about image processing in my next blog post.]

Flight altitude was 50-60 m. Images were taken using a Canon S110. The size of the survey locations ranges form 5 to 25 ha (12-60 acre). The length of the entire area covered by the orthomosaics (from A to F) is 2.7 km (1.7 miles).

The maximum spatial offset between the orthomosaics is 1.2 m (3.9 ft). The average is 0.87 m (2.9 ft).

Overlay   Offset
A vs B    0.8 m / 2.6 ft
A vs C    0.6 m / 2.0 ft
B vs C    1.2 m / 3.9 ft
B vs D    0.4 m / 1.3 ft
D vs E    1.1 m / 3.6 ft
E vs F    1.1 m / 3.6 ft

Based on my previous tests I expected a much lower accuracy of about 2-4m maximum spatial offset. But it seems that both, the M8 GPS as well as the image processing pipeline, are pretty accurate and stable.

My next tests will be on RTK post-processing as well as the (promised) bench comparison between different M8 versions and antennas.

Best regards,

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  • I'm really interested to hear about your RTK setup. Specifically, if you're able to get the Pixhawk to record RAWX messages from the M8T for later use in post-processing and if the M8T is the lone GPS on board, supplying positional information to the pixhawk while also recording carrier phase data, thus eliminating the need for a dual GPS setup.

    Nice work as usual!

  • This research is awesome! keep up the great work

  • MR60

    Thx for posting these results. I suppose you did not do a comparison with the standard 3DR GPS+compass module (LEA6 or NEO7) ? I have read some conflicting reports about the fact the M8 is only giving an illusion of beter precision versus NEO7 or LEA6, and I would like to see trustful test data for an objective comparison.

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