.. UHDAS+CODAS documentation master file, created by sphinx-quickstart on Wed Jan  7 14:24:39 2009.
   You can adapt this file completely to your liking, but it should at least
   contain the root `toctree` directive.

Heading error causes cross-track errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. toctree::
   :maxdepth: 2



An error in heading or transducer angle will result in a cross-track
bias in the ocean velocity.  A good rule of thumb is "one degree error
results in 10cm/s cross-track error".  Since typical open-ocean velocities
are often 20-40cm/s, this constitutes a large fraction of the signal we're
interested in measuring.


.. image:: cross_track_error.png
   :alt: ADCP data: cross_track_error
   :scale: 50


Classic examples include

- ADCP was re-mounted at a different angle from earlier, or a new
  installation goes in with an unknown transducer angle:  In these
  cases, resurvey with bottom track or reciprocal track, reset the
  transducer angle, and start again with a new cruise segment.

- accurate heading device failed, and a segment of data was processed
  using the reliable heading device only:  The solution here depends on
  the length of the gap and whether there is a suitable substitute
  for the primary accurate heading device.



Follow `this link <../../../codas_doc/dataviewer/phase.html>`_
(from "gautoedit" documentation) to see the effect of a
rotation error on the ocean velocities.  A difference of 4 degrees
is enough to swing all the ocean velocities from port side to
starboard side.

.. NOTE:: A one-degree error will have the same effect on ocean velocities, whether it comes from an error in transducer angle or an error in heading. **RULE OF THUMB** one degree error in heading is .1m/s error in cross-track velocity.

**the riddle**

The next two pictures show the real ocean.  But are those
swings of ocean velocity from port to starboard real? or
are they caused by a heading error?  An error of 1 degree would
be enough to do this, and it is essentially in the
cross-track direction, so it is consistent with a heading error.


.. image:: os75bb_vect.png
   :alt: ADCP data (vector plot): eddies? or heading error?
   :scale: 50


----

.. image:: os75bb_cont.png
   :alt: ADCP data (contour plot): eddies? or heading error?
   :scale: 50


Because we know the heading is calculated using an accurate
heading device, and that heading device was working, this is actually
a representation of ocean currents, not an artifact.