I have vessel heading, angle and distance to the animal sighting (С) and distance to point B. I need the coordinates for both points C and B.
How do I go about this for many marine mammal sightings in QGIS?
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function project: Returns a point projected from a start point using a distance, a bearing (azimuth) and an elevation in radians. Use project get B then again to get C.– SethinacanCommented Jul 23 at 7:22
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If the coordinates are angular (e.g., WGS84 decimal degrees), then this is just the First Problem of Geodesy, and can be solved by a library that supports the Forward (aka Direct) function.– VinceCommented Jul 23 at 11:59
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What should I do when someone answers my question?– BeraCommented Jul 28 at 17:30
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1 Answer
Here's a layer (New Scratch Layer) with three points with an attribute table. There are two bearings and two distances. This is your A data, which should look something like this, right?
Using the "project points" tool from the toolbox and setting the bearing and distance to come from the fields "bearing" and "distance" returns the Projected layer above the Scratch layer. This is your points B.
Here's the dialog for that. Note the yellowed drop-downs where I've overridden the constant value with the field name.
B still has the d2 and b2 attributes from A, so we now run "Project" on that layer, and specify the distance and bearing to come from those fields, resulting in the Projected layer at the top, which I should rename C.
Note it keeps all the attributes from the A layer, so if you have times or species IDs etc they're all preserved.
This all assumes your directions are all angles from grid North, and not relative to the boat heading or anything like that. If magnetic bearings you will need to correct to grid north first. You will also need to transform to a grid coordinate system if you have lat-long data. Also you may need to flip some bearing directions (ie use 180-bearing) if your directions are inverted. Hard to tell from your diagram because that boat doesn't look like its really going East. Some sample data always helps with questions here (even if its not real data, similar simulated data is very useful).
