Having recently bought a Nexus 7 I have been keen to explore its suitability as a tool for archaeology. First a test of its mapping potential using GPS.
For this test points were recorded on the tablet using the Droid2CAD app. First 100 points were recorded using a Qstarz GPS818X bluetooth GPS receiver running through the Bluetooth GPS Provider app. 100 further points were recorded with the Nexus 7s own GPS. All the points were recorded in the same location; indoors sat on the sofa, just to challenge the sensitivity of the receivers. The data sets were recorded on subsequent evenings over an hour or so.
The graph above shows the range of accuracy readings. The scale on the left is showing the accuracy in meters. The top red line is for the on-board GPS and the lower blue line is for the bluetooth GPS. The difference in quoted accuracy is dramatic, however when compared to the plot of the latitude and longitude below the results are not so different. The circles are from the bluetooth GPS and the triangles for the on-board.
Both sets of points have a similar clustering within a 10m radius, although the actual center of this area is out by about 5m west and 2m north of its true location. This is likely to be an issue with converting the native GPS WSG84 coordinates to OSGB1936 in QGIS. Within that 10m circle the Qstarz GPS818X has 49 points with a tight cluster of 15 readings within a 2m diameter circle. In contrast the on-board GPS has 73 of its points in this area with no apparent clustering.