docs(misc): add info on snap_threshold and crs

Rephrased the info that was posted in #22.

docs_src/misc.rst

@@ -36,3 +36,31 @@ geometric results. Specifically, use of ``Validation`` will remove
 Z-coordinates to avoid unexpected behaviour. Use of ``Network`` will by
 default remove z-coordinates but this can be disabled (See ``Network``
 input arguments)..
+
+Snap threshold parameter
+------------------------
+
+Both in validation and analysis a ``snap_threshold`` parameter is used. The
+``snap_threshold`` is for the most part designed to handle the (unavoidable)
+errors in topological snapping. If the traces in the map are snapped using the
+snapping functionality of e.g. QGIS or ArcGIS, the distance between endpoints
+and the segments they should be snapped to be should be well below the
+threshold of 0.001, which is the default value used in ``fractopo``. This
+threshold during digitising can probably be changed in the settings of these
+software but for the most part the snapping error should not be connected to
+e.g. the lengths of the traces you are digitising.
+
+I would recommend, rather than using a higher ``snap_threshold`` in
+``fractopo``, to make sure that the snapping is done properly within the
+GIS software used in digitising.
+
+Coordinate systems
+------------------
+
+``fractopo`` for the most part assumes a metric coordinate system and
+has mostly been tested with data which has the coordinate system of EPSG
+3057. Issues with data from other coordinate systems should be posted on
+*GitHub*. Using a coordinate system with metric units should be the
+first step in debugging if you believe an issue is caused by
+``fractopo`` not handling other units correctly. Especially the
+``snap_threshold`` value should be adjusted based on the units used.