aebrilievi
Second Officer
Italy
Offline
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Sorry, non native english here.
I disagree with LV_Forestry when he said "It does not matter ! What matters is GSD, Ground sample distance. ".
From what i understand, Candarco need a DSM suitable for terrain avoidance, is it right?
From my experience, for the terrain avoidance function the Matrice 300 need a DSM in EPSG 4326 (wgs 84) with ellipsoidical height.
Google earth give you geoidical height (A.M.S.L.),
The Ellipsoid is a simple geometrical shape (a sphere stretched at the poles) to approximate the earth.
The Geoid is an irregular surface, affected by gravity irregularity (heavy rocky masses under the terrain, for example, can "deviate" the gravity vector).
Every traditional topographic gear give you height on the geoid surface, because is leveled with a toric or spheric liquid level; the gps system can't; he can only give you an eight above a geometric reference, the ellipsoid (or datum).
The height difference depend in what part of the world you are; here, in north western Italy is about 45 meters
When i need a terrain avoidance, i follow two paths:
When i need the maximum accuracy, i fly a preliminary mission with a constant height that guarantees safety, generate a DSM (WGS84 with default elevation reference in DJI Terra for example), next i put this DSM in the controller and subsequently i can fly the "real" mission at reduced elevation, with the Matrice following the DSM elevation (plus, obviously, the elevation i set for the mission).
When the main mission isn't so low on terrain (for example when i must map rolling hills or mountains territory at least at 50 meters elevation), normally i download from italian local administrations a portion of DSM (generally more accurate than GE), then i convert in EPSG 4326 and geoid elevation to ellipsoidical elevation using Terrascan or similar capable softwares; next i put this DSM in the controller, in the field i check if the elevation delta is acceptable and finally i fly my mission.
Probably, when you need to fly not so high in difficult terrain, the "double mission" approach is the safest. First an highest mission to obtain a reliable DSM in correct coordinate/elevation system, and then the main mission above the DSM.
Sorry for the long post and for my english. |
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