Georeferencing

GIS datasets are usually created specifically for a GIS system complete with geographically referenced coordinate data built in to the file. This is important because these kind of data sets are lined up properly automatically. However there will be times when the data sets do not have coordinate information readily usable by the GIS program. These kinds of datasets usually include digital scanned copies of paper maps and aerial photographs before GIS was used. In this session we will cover the different methods used to input the geographic referencing to an unreferenced dataset and to create geographic data from non GIS ready maps and photos. Georeferencing is the process of making a raster dataset of an aerial photography or digital scan of a paper map to be lined up with a referenced area. To initiate this process we firstly start by adding the unreferenced raster and the referenced raster to the data frame. Then use the fit to display option on the same georeferencing tool to bring the layer you want to reference into the current area that you are viewing and manipulate your display so you can clearly see features on both the known and unknown layers. Secondly you must identify common points from both the referenced and unreferenced rasters and add points to line up both datasets; this is done with the add control points function. First click on the feature on the known area and then click on the same feature on the unknown area. This is process is made easy by adjusting the transparency of a layer; this option is accessed in the layer’s properties. The map is lined up better when the points are evenly scattered. The residual option allows you to view each point added and the total RMS errors. After sufficient points are added the georeferncing needs to be updated so that the data is saved. Secondly the same steps are followed to align the second part of the unreferenced dataset to the referenced dataset and the georeferencing is updated. Both unreferenced datasets must be properly aligned and must be in the 1^st order transformation. The third step is to update the referenced datasets with new feature that were not displayed; these features are a road, a new building and athletic fields. To display these features we used knowledge learnt from the previous sessions to start an editing session to add the new road and the new building with the use of a circle in the middle of the building. For the athletic fields a new shapefile was created to display the four athletic buildings. To finalize the map basic map elements need to be added which are title, scale, legend, north arrow, for this map we also displayed our RMS errors.