Notice: I was unable to post the following article on the DJI Support Forum, but thought that this article may still be of interest to Pix4Dmapper users as well, though the issue discussed pertains specifically to bogus altitudes written to the EXIF by DJI’s Inspire air frame.
Our second test flight for survey grade aerial mapping may have seemed like a bust given that we’ll need to take another go at it; however, the lessens learned from this begin with visualizing in Pix4D the importance of well placed surveyed ground control points. In the image above, we are viewing towards the west at about normal eye level when standing level on the road surface. The three blue precisely surveyed points are in the distance beyond the limits of the flight. I believe that this height anomaly is largely or entirely attributed to the enormous difference between the geotagged camera altitude and its true altitude.
Inspire 1 Firmware v.126.96.36.199
Crazy east-west flight path due to the DroneDeploy app’s limited functionality (v1.4.7) after Pix4D Capture failed to process flight path.
In addressing this issue yesterday after we returned from the field, I called DJI tech support (1.818.235.0789) and eventually spoke with Mike on both occasions, who in both instances supposedly tried to connect me with the so-called tech support for the Pro product Inspire as he was admittedly unqualified to give technical support for the Inspire. In both instances, the signal immediately changed to a busy signal. Mike refused to provide a direct phone number to that DJI department when asked during my second phone call.
Next I tried posting on the DJI forum after having first successfully registered and signing in; however, am thwarted from posting: "
Hi, Kelly Bellis.
An email has been dispatched to email@example.com with details on how to active your account. Please check your email and follow the link in that email before you can post on DJI Forum.
Haven’t receive the email? Please click here to send the email again.
Even after clicking to send email several times, no such email has been received on my mail server. Had I been successful in posting, here is what I’d post and for informative purposes am posting on the Pix4D support forum. Also cc:'ing to firstname.lastname@example.org:
Altitude off significantly - What’s up with that?
Noting EXIF field names and values for image [DJI_0514.jpg]
Airframe: DJI Inspire
The geotagged images produced by the DJI Inspire are in reasonable agreement horizontally with the precisely surveyed ground control; however, the recorded altitude of the camera (FC350) is quite a bit off; e.g., one nadir image (DJI_0514.JPG) reports the altitude of the camera at -47.4 meters or about -155.5 feet below sea level, whereas the actual ground elevation was about 112.7 feet NAVD88 (Ellipsoid height near this location was 28.1 feet). The flight plan was supposedly at an altitude of 150 feet above ground, making the actual elevation of the camera at about 262.7 feet NAVD88.
If we assume, for the sake of discussion, that in theory, _ zero _ altitude to be determined at ground level of the location where take off occurred, then the algebraic sign recorded in the geotagged image looks like a possible chief suspect as being incorrect.
The Inspire’s User Manual (dated 12/17/2015) v2.0_1218.pdf offers a little in terms of only suggesting how the altitude is derived for geotagging. It states (on pages 12 and 28) that height above ground; i.e., altitude, is barometrically linked. On the other hand, if the height that gets recorded is based on GPS, then there is way too much error, particularly given the wide open unobstructed view of the sky and 11 SVs all easily within the Inspire’s line of sight to them.
Has anybody else run survey grade ground control and observed similar errors in geotagged heights?
In the EXIF data of the photo, there should be clear indication that altitude is neither determined by nor associated with GPS, and that it is solely based upon the device’s internal barometric reading. Furthermore, there ideally should be an EXIF field and value for GPSEllipsoidHt once a 3D fix is known.
Mr. V. Kelly Bellis, ME PLS 2099 :: WQTS485
17 Union Street :: Ellsworth, ME 04605