1 - Georeferencing a raster dataset using a Basemap Layer

Georeferencing a raster dataset using a Basemap layer

Raster data or digital images don’t usually contain an adequate spatial reference information, they cannot be immediately integrated with other GIS data. So, once the data are input, they need to be assigned their proper map coordinates, and aligned properly with existing geographically referenced data.

In order to use these kind of data, we should define their location using map coordinates and assign a coordinate system. This process is called Georeferencing. It relies on the coordinates of points on the scanned image with points on a geographically referenced data by linking points on the image with those same locations in the geographically referenced data.

The easiest way to georeference your data, is to use Basemap Layers as a geographically referenced data. The Basemap Layers are based on a powerful drawing engine to provide a visual context on which you draw your operational data.

To make it done, we will need two layers:

• -          The image you want to georeference
• -          The geographically referenced layer (the Basemap layer)

We will need to display the Georeferencing Toolbar:

-          Choose ‘Customize’ from the menu, then ‘Toolbars’, and select ‘Georeferencing’.

Remember : Internet connection required to display the Basemap Layer.

• 1.       Start ArcMap
• 2.       Add the layer dataset you want to georeference
• 3.       Add the image you want to georeference ;
• 4.       Create a Bookmark to navigate at ease ;
• 5.       Start Georeferencing ;
• 6.       Assigning a coordinate system

For more explanation :

•   Start ArcMap
•  Add the layer dataset you want to georeference ;
• Select ‘Add Data’ from the ‘File’ menu, or from the “Standard” Toolbar. Then choose ‘Add data’ and “Add Basemap”. For this recipe we have to choose ‘Imagery’ Layer.
•  Add the image you want to georeference ;
• Use ‘Add data’, then browse to your file, choose it and validate by clicking on ‘OK’. If ArcMap do not display your image, right click on its name in the “Table of Content” and choose ‘Zoom to layer’.
  
  
  • ·         Create a Bookmark to navigate at ease ;
  • Zoom to your raster dataset.
  • Choose “Bookmarks” in the menu, then “Create Bookmark”. On the “Create Bookmark dialog box” enter a name for your first extent, for example ‘Non georeferenced image’.
  • Then zoom to the same location in the Basemap Layer, and choose “Create Bookmark” again. Rename it.

  
  

  • To manage your Bookmarks, choose “Bookmarks” in the menu, then “ManageBookmarks”.
  •  Start Georeferencing ;
  If we have many layers in the “Table of Content”, we should choose the image from the Layer drop-down arrow in the ‘Georeferencing’ toolbar.
  
  
  
  
  Double click on the bookmark that we have created to zoom to the image. Then pan to the area where you want to insert your control points.
  
  

  Use the ‘Add Control Points’ button in the ‘Georeferencing’ Toolbar to select a known location in both non georeferenced image and the Basemap layer. Then pan to the same location in the Basemap layer and click on it.
  
  

  Repeat the previous step until you reach enough links for the type of transformation you will apply. In this case of projective transformation, we will need 5 links. 4 links in the corners and one link in the center.
  
  

  In the ‘Georeferencing toolbar’ click on the ‘View Link Table’ button. This dialog box helps us to show links and errors in tabular form. If we are not satisfied with the ‘Total RMS error’, we can delete the link which have the biggest residual value. Then recreate it.
  
  

  Rectify your map: Choose ‘Georeferencing’ in the ‘Georeferencing toolbar’, then ‘Rectify’. This operation will display a ‘Save us’ dialog box. Choose the ‘Output location’, the ‘Name’ of your georeferenced image, the format of your file. TIFF format is highly recommended, to conserve the maximum pixels number of the image.
  
  

  
  

  •  Assigning a coordinate system :

  In ArcCatalog, right click on your georeferenced image. Choose ‘Properties’. In the ‘General’ tab of the ‘Raster dataset properties’ dialog box, choose ‘Edit’ of ‘Spatial Reference’. Browse to your coordinate system, select it and validate by ‘OK’.

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