2. SETTING UP YOUR PROJECT
2d. Georeferencing a field map
2d. Georeferencing your field map
Right now, your scanned field map has absolutely zero geospatial data associated with it. In order to properly digitize it and work with data, you need to give your map some spatial information. This process is called “Georeferencing”, and is actually quite simple if done right.
You did remember to label coordinates of known locations (control points) on your map while working in the field, right? Right? Good.
To georeference your field map, follow these steps.
1. Open up the “Georeferencing” toolbar. Customize –> Toolbars –> Georeferencing.
2. Add in your scanned field map using the same 3 steps as above from “Adding your first bit of background data”.
3. Whoops! You have an error message:
This is totally okay! We haven’t assigned any spatial reference data to your field map yet. Click “OK”. Because Arc doesn’t know where to place your map, it arbitrarily assigns coordinates to the image based on its dimensions from the scanner. Therefore, the data you just loaded in to your project probably won’t be visible in your view just yet.
Right-click on your field map in the “Table of Contents” to get the following menu:
There are a lot of things that can be done here using this menu, we’ll discuss them as needed. By selecting “Zoom to Layer” from this menu, ArcMap will automatically zoom your view to the extent of your field map (or whatever other layer you have selected). Go ahead and do that now.
4. Now your field map should be visible in the map window. This next part relies heavily on the Georeferencing Toolbar. Check to make sure that the “Layer” selected is your field map (or the layer that you’re interested in georeferencing). Otherwise you’ll assign bogus spatial information to another layer! This common error has been known to make many people very frustrated, so let’s start out by alleviating it altogether. Click on the “Add Control Point” icon in the Georeferencing Toolbar. Look at the following figure to see where these various icons are:
Next, navigate to one of your control points on the field map. It is very helpful to zoom in to that control point for accuracy and precision when georeferencing. Left-click to establish the control point, then right-click to “Input X and Y…”. A new window will open, and here you’ll input the X and Y (easting and northing if using UTM) coordinates of that point. Click “OK”.
5. Your map should disappear from view. Not a problem! You’ve just assigned it some spatial information, so Arc moves the map so your control point is in its correct geographic location. Now, simply right-click on your field map in the Table of Contents and choose “Zoom to Layer” to get to where your map is.
6. Repeat adding control points. A good number to have is 3, but 4 (i.e. the corners of a square) is pretty ideal. If you goof and add a point you don’t want, or mis-type a number, click on the small table icon in the Georeferencing Toolbar to edit your control points. Be mindful that if you delete all but one, Arc will discard the spatial information and you’ll have to start over.
NOTE: You can also use background data to georeference a field map, provided that (1) the background data is georeferenced properly; and (2) there are EXACT points on your field map that can coordinate to the EXACT SAME point on the background data. Using the “Add Control Point” tool, first click on the known point on your field map, then a second time on THE SAME POINT on the background data. Do this for a minimum of 3 or 4 points or until you map is properly georeferenced. While doing this, you will likely need to zoom around your data, the easiest way to do this is using the “Zoom to Layer” option.
7. Once you’re satisfied with the spatial position of your field map, there’s one more step. In the Georeferencing Toolbar, choose Georeferencing –> Rectify or Georeferencing –> Update Georeferencing to save your map with the geospatial data you’ve assigned to it.
IMPORTANT!!! If you don’t “Rectify” your map you’ll discard all the spatial information you just gave it, so the next time you open up Arc you’ll have to repeat the georeferencing process.
The “Save As” window will pop up, and you’ll need to adjust a few things. First, make sure the “Output Location” is your \arc directory or project folder. Second, change the “Format” to “TIFF” from the drop-down menu. Lastly, give your georeferenced map a new name, making sure to follow the Rule of Names.
8. Arc doesn’t automatically load in your new, georeferenced map. Remove the original map from the Table of Contents (simply un-check its box, or right-click –> Remove), then add in the georeferenced field map as a new layer.
Now is another good time to save your project. Actually, any point while working in ArcMap is a good time to save your project. I’ll let you decide from here on out.