[Talk-GB] Surrey County Air Survey

[James Rutter]

Guys, there's been some suggestions on the list that there are discrepancies
between the new air survey and previous GPS traces and also suggestions that
the image needs 'reprojecting' etc.

The image is produced by state of the art technology. As someone observed it
is made up of thousands of individual frames. They all look as if they have
odd alignments (if you look at the tiles around the collar of the image)
because this is what happens during aerotriangulation and block bundle
adjustment of air survey frames to compensate for the pitch, yaw, roll and
crabbing of the aircraft as it's flying the survey....it's not made up of
precise squares and 90 degree right angles!!. There is some extremely
expensive kit on the plane called the IMU (inertia measurement unit) and
military grade GPS (which you need a license to use!) so the air craft knows
precisely its position and attitude when each frame is shot. Independant GPS
ground control also has a bearing on the image which is 100% map accurate
and also fully orthorectified. The image is very high specification (local
authorities don't buy rubbish!!) and is in full use by Surrey County and the
districts.....any discrepancies in the accuracy and the image would not have
been accepted....and don't forget that we're checking the image accurace to
1:1250 scale Ordnance Survey Mastermap. My polite suggestion would be that
OSM data alignment is the issue, not the postition of the air survey.

James
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