Opened 9 years ago

Last modified 9 years ago

#290 new flight processing

Create a recommendations document for ground teams

Reported by: mggr Owned by: benj
Priority: whenever Milestone: The Glorious Future
Component: Support Keywords:
Cc: Other processors:

Description

Should include planning for hyperspectral, lidar, etc.

Change History (1)

comment:1 Changed 9 years ago by mggr

(oops, sorry you caught this, Ben)

Some text that might be useful from an email to Barbara (Afar, Ethiopia campaign):

In our processing so far, we've found the horizontal accuracy of the
LIDAR to be very good (at least in the UK) - the main thing with
additional ground control points is to pin down the height, which is
more likely to vary.  We will also use anything we can to double check
the horizontal accuracy is good.

Barbara wrote:
> There are continious GPS stations in the field with an antenna of about 30cm surrounded by some sort of stone wall about 30cm high. Do you think they could be picked up by the lidar? Also I'm not sure how many of these stations are directly in the survey area.

Those sound useful but may be difficult to spot as the LIDAR point
spacing will almost certainly be much larger than 30cm.  If they're in
the survey area and we can see them we'll definitely try to make use of
them.

To give you an idea of the resolution we'll get, the LIDAR spot size is
about 22cm at 1000m, increasing with altitude.  Point spacing will
similarly be affected by altitude, operating and ground conditions, but
hopefully we'll have 1 point per meter.

> So if it's preferable to get some additional points how (array?) and where do you suggest I get them since there will not be many very obvious features such as houses etc in the area. Also what would be the minimum accuracy of the points which you would require. Since I only have limited time in the fiel to make these additional measurements.

As regards accuracy, we'd need GPS fixes to be accurate to <20cm
horizontal and <40cm vertical in order to improve the basic accuracy
we've seen.  These can be obtained from a local GPS measurement plus a
fixed base station (differential GPS) or by taking a long fix on an
independent GPS (really needs to be several hours..).  If there are no
fixed stations nearby, you could also take quick local GPS fixes and set
up your own base station, leaving it running for ideally 24hrs and
covering the time when you make your local measurements.

There are two sorts of additional ground data that'd help us.  In both
cases, putting the GCPs near your areas of interest is best, though I
remember you had very large areas - if so, whatever is possible is fine.
 If you have overlapping flight lines we can transfer the positioning
from one to another so you don't need too many points.  If they are
non-overlapping, it would be helpful to have GCPs in each separate
section.  If the lines are very long, it would be useful to take a
couple of GCPs along the lines.

For checking/fixing horizontal (x/y) accuracy, GPS fixes on one or two
targets easily visible from the air (a corner of a house, the inner
corner point of a road junction, etc).  Objects will need to be 5m+
across to be easily visible and to give us a fair chance of pinning down the point you've got a GPS fix on.  If you have problems finding a suitable target, you might be able to use an artificial one.  It would have to be large and reasonably flat - perhaps a tarpaulin or two making a X?

For checking/fixing vertical (z) accuracy, a sequence of (say) 5 points
spaced about 5m apart along a straight line on a hard flat surface
without nearby covering vegetation.  A tarmac road is ideal for this if
there's one nearby, otherwise flat rock surfaces, hard dirt, etc.  We
need a series in order to make a line to compare against the LIDAR
surface - single points can include too much noise or may get confused
by a single noisy LIDAR point.

Snapping a quick photo of the general area would also help us locate
things or help show up any funnies about surfaces near the GCPs.
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