Map A before

Map A after

Google Earth

What looked like a path across a cultivated field is in fact a path between private land surrounded by a hedge and fenced private land. Runners running North/South were expecting to hit a path running West/East between controls 35 and 47. Given these paths are permissive routes over cultivated land (which is otherwise out of bounds), this caused a lot of confusion.

Map B before

Map B after

Google Earth

Start was described to be in the middle of two hedges and this matched the view in Google Earth. In actual fact, significant development had taken place and one hedge had been cut back to make way for a new road into a parking area for a new building. This discrepency was noticed during an on-the-ground survey.

To check the controls in Google Earth, you do not need to load the KMZ file (which contains the map) as you are aligning the controls with the aerial imagery, not with the map. Use Purple Pen to align the controls on the map (so that the printed map is correct) and aerial imagery on Google Earth to align the hit points of controls with the reality of what is on the ground. Beware, the aerial imagery may be some years out of date, so if your controls are in a newly-built housing estate, for example, you may only see a muddy field. If this happens, there are a number of ways to deal with it:

• Use an alternative source of aerial maps, e.g. Bing or the various options available in Open Street Map. Find the point and note its latitude and longitude which you can enter into Google Earth when editing a point
• Use a GPS watch to visit the location and note down the latitude and longitude (or save a lap position). Enter these into Google Earth when editing a point
• Your KML files that you originally converted from your IOF KML and KMZ will have had points in approximately the right place. If you found yourself adjusting each point by roughly the same amount, do the same to your control which has no aerial imagery to compare to

As you go through the controls, you should also compare the map in the vicinity of each control (and the start/finish) with what is actually on the ground. Take some time to treat the map with a critical eye. Remember that Open Street Map is crowd-sourced data, so there is no guarantee of any accuracy and what is important to the person who edited that part of the map before (e.g. the alignment of the boundary between their garden and their neighbours) may be very different to what is important to an orienteer (e.g. which side of hedge the path is on). For a similar reason, areas away from residential areas may be more vague. Paths through woodland may be represented as simply a straight line (if you want to get them right, you can take a GPS trace and use that as a template to fix the paths). Hedges, ditches and boundaries of fields may be only a rough approximation.