Exercise: Measuring point coordinates using WMS Google maps

Coordinates are often needed for ground control points (GCP) with the objective to georeference remote sensing data or scanned maps without graticule. Google maps can be used as one source for registering point coordinates.

Import highly accurate DGPS points stored in a ASCII CSV file

1. Click Add delimeted Text File. Select the input file GBData\GNSS\Festpunkte_Forstbotanischer_Garten.csv. Define X field: East and Y field North. OK.
2. Specify the coordinate reference system (CRS) ETRS / UTM zone 32N. Search for the EPSG code: 25832. Three precise geodetic survey points are loaded into the viewer. The current active project CRS is displayed on the lower left frame of the viewer as EPSG:25832.

Use Google maps as background layer

1. Activate Web --> Openlayers plugin --> Google Maps --> Google Satellite. A high spatial resolution aerial othophoto originally aqcuired in 2009 by GeoBasis-DE/BKG (Bundesamt für Kartographie und Geodäsie, Frankfurt and provided as web map service (WMS) by Google is displayed in the viewer. Note, the active CRS is automatically changed to EPSG:3857 (WGS84 / Pseudo Mercator).
2. Label the geodetic points. Layer --> Properties --> Layer --> Labels. Switch to Show labels for this layer in the drop list and Label with the field ID. Change text size to 12 and text color to white.

1. Zoom close in to a geodetic point. What do you notice? The Google based layer is misaligned when zoomin in to about 1:1000 scale! find out the maximum zoom level where the Google layer is still correctly displayed.

Display GNSS points of a long term measurement before and after differential postprocessing

1. Add uncorrected points of a long measurement using GNSS receiver Trimble GeoExplorer 6000. Layer --> Add layer --> Add vector layer open the shapefile \GBData\GNSS\PosPkt.shp. Zoom in to point VP1 and observe the scattered point cloud. Measure the extent of the GNSS point using the Measure line Tool.
2. Now open the GNSS points as a result of a differential postprocessing: Layer --> Add layer --> Add vector layer and open \GBData\GNSS\PosPkt_DGPS.shp. Measure the extent of the postprocessed GNSS point using the Measure line Tool. Which horizontal accuracy can be expected?