Export of annotation locations

[tschoeni]

While reporting could you please compute the latitude & longitude of each annotation? You report image_lat/lon and px_x/y but from the known image resolution (in m^2) and eventually the GPS_heading you can compute px_lat/lon as well.

[mzur]

Can you describe how to do that, since the images are not squares (and you probably have done this before)?

[tschoeni]

From the pixel resolution of an image and the area you can compute what the side lengths are in meters and thus have a pixel / meter ratio for an image. To combine it with the lat/lon of the image you have to make some assumptions: 1) The seafloor is flat and the camera was oriented perfectly perpendicular to it. 2) The lat/lon coordinates (img_lat,img_lon) correspond to the top left corner of the image.

You then need to convert image size in meters into image size in degrees (which depends on the location, see https://en.wikipedia.org/wiki/Decimal_degrees). Combined with the known meter / pixel ratio you can then compute a scaling factor from pixels to degrees (px_to_degree).

The Pixel coordinates are then:

px_lat [°] = img_lat - px_to_degree * sqrt(x^2 [px^2] + x^2 [px^2]) * sin(atan(y [px] / x [px]) - heading [°])
px_lon [°] = img_lon + px_to_degree * sqrt(x^2 [px^2] + x^2 [px^2]) * cos(atan(y [px] / x [px]) - heading [°])

where x and y are the pixel coordinates within the image as stored in annotation_points, heading is the direction of the camera (0 if the x axis of an image is parallel to the equator, 90 if the x axis of an image is parallel to a meridian and the direction of movement is eastward).

This all depends by a fair amount on coordinates, +/- rules and degree systems (radians vs degrees! make sure the library used to compute sin / cos / atan provides and uses the correct values).

[mzur]

This might be resolved with biigle/geo#18.