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Inconsistent Elevations

I have done considerable test with the app and I highly suggest that you ditch using the barometric altitude and start using the ellipsoid altitude from the GPS. I am a Professional Land Surveyor and the accepted practice for land surveying is to use the ellipsoid elevation and apply a geoid separation to compute MSL elevations based on the geoid model used.

Whatever is being reported after the last firmware upgrade IS NOT MSL. in three flights that I flew (all within 2 hrs) There was a variation of 250-285 ft from actual MSL. While these can be compensated for after the fact, it would be considerable more consistent with GPS elevations. I have tested this by placing a GPS tracker on the drone and downloading the log file. Accuracy was less than 1.5m.

Another giant issue is the the ground elevation changing between battery changes. Yesterday, the difference between a battery change mission was 4.6(m) in the ground images and difference in the flight images between flights was an average of 5.3(m).

Now I am sure that you are asking why should you believe what I am saying. I have been using several different drones and selling and supporting the senseFly eBee for two years. All of these drones use the GPS elevation or commonly referred to as the ellipsoid height.

Now I know that you are affiliated by Maps Made Easy, But there are many of us out here that use other software and we are only looking for consistently geotagged images. Please consider making these changes. This would make you app a much better program for all users. I would be more than happy to help you with this issue.

Mark Paulson

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We don't have access to that information. We get either barometric relative altitude or GPS altitude from the DJI SDK and the aircraft. The GPS altitude has some really big altitude errors and basically can't be trusted.

Relative altitude is the best measurement that is available and we base our processing off of that. If you include a ground reference image with your upload, we use that to take the location of where the aircraft took off and get a value from the NASA SRTM database and use that as the offset for the rest of the images. Feel free to project it from the used WGS84 ellipsoid.

For more information on the SRTM data, read here: https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission

We cannot control what is reported by the firmware. We have no control over what values get written to the images. DJI seems to change it at will without telling anyone ahead of time or documenting that it happened. We have addressed the issue within Maps Made Easy by adding the workaround that is described above.

The method above works across multiple battery changes.

Tudor
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Hello Mark;

I am facing the same issue starting from FW 1.5.30 (P3P) whereas the GPS Absolute value (ASL) is not reliable at all while the Barometric Relative value (AGL) is good and reliable in a certain period and weather conditions.
In fact, I am also getting an error around 100 m sometimes (Gnd Pic) and around 7 m between pic in the sky !! I contacted DJI team for that case and seems nothing has been changed yet.
Anyway; please let me know if you know a standalone Software that is able to Automatically replace the written Absolute values (ASL) in the EXIF files (Metadata) with the Relative ones (AGL) since both values are written in each picture.

Regards;
Zahi Khawand

Zahi Khawand 0 votes
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Tudor, I have to respectfully disagree with your assessment of The DJI Barometric altitude> I flew my test area with an eBee RTK UAV. I also shot actual elevations in the field with a Leica GS 14 and field verified the elevations. I then processed the Map Pilot images from the P3p with Maps Made Easy.

Next I used the DEM from MME using the P3P images and the DEM from the from Pix4D and the eBee images and created a profile line in the same location for each DEM here is a link to the results:
[IMG]http://i68.tinypic.com/6eqcqt.jpg[/IMG] Hardly acceptable. Also, there was a 6 meter difference between battery changes. I really think you would be much better of using the GPS ellipsoid elevation.

Mark Paulson 0 votes
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@Zahi. I know of nothing that will do what you want automatically. You might look at expert GPS, but I haven't used it in several years.

Mark Paulson 0 votes
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As Zahi noted, we have to be careful with what numbers we are talking about here. Previous to FW 1.5, DJI reported the relative altitude as read by the barometric sensor in the regular GPS section of the EXIF data. For FW 1.5 and up, DJI has opted to populate that same element with the GPS derived "GPS ellipsoid" value. The barometric sensor relative altitude (and a bunch of other fun stuff) is buried deeper within the file but most software can't dig it out.

The GPS derived altitude value is likely what you are seeing as having the 6 meter offset between batteries. The barometric sensor gets zeroed out while it is on the ground and is very accurate for making relative measurements over a short duration like drone flights.

As long as the relative altitude values are available and there is a valid ground reference image, we use it. This helps us normalize the altitudes between flights by having a more consistent reference and lets us use the SRTM database as a known point to serve as an offset for the relative values.

Clearly the DJI GPS cannot hold a candle to that of the eBee (at 10X the cost), but using the SRTM data at least allows the values to be consistent and be closer to the above sea level value.

The curvature in the line is an artifact of the equally bad GPS Lat Long error which can be made better with more overlap (more samples).

If you want to run the tagged Leica data through our system as a comparison we will refund the points.

Tudor 0 votes
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We have added a new tool for converting the bad GPS altitude data that gets written into the GPS Altitude EXIF value to a corrected value based on a user provided elevation or ground reference image.

https://www.mapsmadeeasy.com/tag_fixer

This will allow for people not using our Maps Made Easy processing system to process their data elsewhere while taking advantage of the steps that we do behind the scenes.

Elizabeth 0 votes
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