Contours and Topography
The topography in this mapping is derived from the Shuttle Radar Topography Mission (SRTM) data surveyed by NASA in 2000. In this survey, radar was bounced off the planet surface to provide the raw data to create a height model. Where an area contained many vertical surfaces the radar transmitted by the Shutle was not always bounced back to the receiver in space and this created voids in the height model.
These voids have been filled using computer interpolation at a later date with varying degrees of success. You should be aware that the topography depicted may not be 100% accurate as can be clearly seen on the Aonach Eagach ridge or around Sgurr nan Gillean in the Cuillins.
The Open Street Map project is a collaborative effort from interested parties around the world. The concept is to provide users with the tools to conduct mini GPS surveys and to trace features from aerial imagery to create the mapping data. Whilst the coverage is impressive, if a road has not been submitted by a particpant in the project then it will not appear on the map and therefore some roads (particularly minor ones) may be missing.
The good news is that you can easily rectify this by ensuring your GPS is logging as you drive or cycle the approach roads and you can then submit your survey to the open street map project!