Transformers
============

Transformers are small (or big) transformations that are applied to your GPX
track after it has been uploaded. This allows Fietsboek to provide some common,
simple editing options, without users having to do that themselves.

All transformers are optional and disabled by default. You can enable
transformers when uploading a track on the bottom of the page. You can also
enable transformers for already uploaded tracks in the editing view.

.. note::

    When enabling transformers for already existing tracks, your browser might
    not show the changed track. Try force-refreshing (Crtl + F5) the page so
    that it reloads the GPX from the server.

In other applications, transformers are sometimes called "filters". That term
however has many different meanings (like the filters on the "Browse" page),
and as such, Fietsboek calls them transformers.

Fix Null Elevation
------------------

The *fix null elevation* transformer removes points at the start and end of a
track that have a "wrong" elevation. This helps to avoid issues when GPX
trackers don't have elevation data yet and fill in 0, leading to wrong uphill
calculations and wrong diagrams:

.. image:: images/wrong_elevation.png
    :width: 200
    :alt: An elevation graph that starts at 0 and makes a jump to 165.

Activating the transformer will produce the following track:

.. image:: images/fixed_elevation.png
    :width: 200
    :alt: The same track, but with a fixed elevation graph that starts at 165.

The transformer considers "wrong" elevation to be either points that have an
elevation of 0, or points that have a slope of more than 100% to the next
point.

To fix those points, the transformer will find the first correct point, and
copy its elevation to the wrong points.