Regent’s Park’s inner circle made a good start point to explore routes appropriate for track-logging with some kind of pattern. The route was planned using Google maps service; its vector based maps are simple without excessive visual variables creating clutter; thus facilitating pattern recognition. The produced trail when projected on the map can be seen as a stickman with arms extending left and right in a throwing motion and legs positioned in a manner that indicates throw-assisting movement. With a relative quantum leap of imagination there is a scarf tied around the neck as well. In hindsight, producing this pattern was an ambitious aim; the track was traced on foot, and took a considerable amount of time. Non-anticipated closed paths encountered around the ‘bucket’ area and are responsible for the ‘scarf’ detail. In a perfect world, the long neck would have been much shorter and the circle-head could contain additional detail within.
A.2 Accuracy, error and appropriateness of the map
The GPS track was collected uninterrupted during one afternoon; during data collection the sky was clear with good visibility, and the GPS was triangulating consistently using 4 satellites. The only time that the signal was lost, was due to street narrowness and existence of big trees obscuring the sky (Park Crescent Mews str. with Marylebone Rd). In this case the GPS lost visibility of all satellites, but the track path was not affected as the distance covered under such conditions was very short.
Is fairly obvious that the overall match of the GPS data and the backdrop map is quite good, making apparent that there was a sufficiently strong signal, tracking the route with high accuracy. There aren’t any particular mismatches, and where a multipath is plotted is due to the paths followed to draw the stickman’s hands and legs which were backtracked – thus track-logged twice. Occasionally, people and traffic lights necessitated diversion from a straight route.
The GPS track was collected according to the WGS841 geodetic system; to project on a 2D coordinate system, the track was transferred to ArcView via the GPSi interface and transformed using the OS British Grid2 by selecting the OSGB_1936_To_WGS_1984_Petroleum3 transformation, which allows for a deviation(4) up to 4 metres; thus there is certainly some degree of error since no exact transformation between two geodetic coordinate systems exist(5). During the transformation process a message ‘The output resolution is larger than the input feature class resolution’ was produced, due to the fact that the output geoprocessing tools in ArcGIS 9.2 is 53-bit7.
Finally, to overlay the track, two backdrop maps, one raster and one vector based were used. The scale of the raster map is 1:25,000, allowing the streets to be distinctly visible; both maps were downloaded from Edina. The track log, and the two maps where overlaid using ArcMap. Note that it was not necessary to create a new Geodatabase, and all layers where created by simple opening the Mastermap gz file.
A.3 References
1. A 3D geodetic coordinate system, that comprises a standard coordinate frame for the earth, with a spheroid surface. (http://en.wikipedia.org/wiki/World_Geodetic_System)
2. A system of geographic grid references commonly used in Great Britain.
http://en.wikipedia.org/wiki/British_national_grid_reference_system
3. This transformation uses the parameters recommended by Ordnance Survey for a Helmer(6) transformation; http://geometrybag.wordpress.com/2006/05/03/osgb-transformations-inside-arcgis/
4. A statistic value that represents the amount of disagreement among points with known longitude and latitude in a coordinate system, when compared with a different coordinate system. http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html
5. Welcome to GPS Network. Ordnance Survey – Great Britain's national mapping agency. www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html
[Accessed March 31, 2010].
6. Helmert transformation: The rotation and translation of a network of points relative to the Cartesian axes, while leaving the shape of the figure unchanged;
7. Technical Articles - ESRI Support. Available at: http://support.esri.com/index.
