Updating an old map with new data is a familiar task facing cartographers. That task is simple when you use Avenza MAPublisher and Adobe Illustrator and when the existing map is already georeferenced.
But when you are handed a map whose georeferencing has been corrupted or removed, the task becomes a little more complicated. Fortunately, MAPublisher provides the tools needed to georeference any map, enabling you to import data that precisely fits existing map artwork.
Your challenge, though, will be matching the map projection, scale, and page position of the georeferencing so imported data aligns with the locations of features on the existing map. There’s a quick way to do this, provided you know the projection of the existing map.
Let’s say you have a map of Frisco, Texas that you know was made using a NAD 83 Texas North Central (ftUS) map projection. To begin georeferencing the map, select an artwork object from the map like the city boundary or a large city park. (Pick an object whose shape you’re certain hasn’t changed since the original map was produced; MAPublisher will size the GIS-derived feature shape to the original artwork shape.) With the object selected, open the Artboards panel, click New Artboard, then choose Artboard Options from the panel menu. In the Artboard Options dialog, click Preset: Custom, and then choose Fit to Selected Art. Now the new artboard tightly fits the extent of the selected state boundary.
This new artboard is the magic genie that will help you georeference the map.
With the new artboard active, import GIS data that contains the city boundary. On the new layer Illustrator creates when you import the data, delete everything except the city boundary. This will help MAPublisher fit the imported Frisco boundary to the old boundary in the next step.
Now open the MAP View panel (Window > MAPublisher > MAP Views). Double-click the MAP View containing the imported data. Enable the Perform Coordinate System Transformation and the click the Destination link. Find the NAD 83 Texas North Central (ftUS) projection and select it. Make sure the Active art board field displays the ID number of the artboard you created previously. Click Auto-scale and then OK. The imported Frisco city boundary will conform to the extents of the artboard, which automatically makes it match the map’s old boundary artwork.
As a result, your map is georeferenced and all data you subsequently import will geographically register with the original map artwork.
What happens if you don’t know the map projection at the outset?
By trial and error, you can use the method described above and change the map projection until you find the one that matches the old map. Or you can use Avenza MAPublisher’s terrific Georeferencer tool.
When you use the Georeferencer, you create a relationship between the page locations of non-georeferenced features and their real-world locations. To do this, plot at least four points on a non-georeferenced layer using the MAP Locations tool. (The more points you plot the more accurate your results will be.) Select the Georeferencer tool and in the Georeferencer dialog, choose a method to map your plotted points to their real-world locations. The three methods include:
- an online map service like Google Maps (shown below),
- keying in location coordinates,
- using MAP World Locations from an open document.
After completing one of these methods, the Georeferencer tool displays a list of map projections ranked by best fit. Pick a highly ranked projection and MAPublisher will reproject the artwork. If you picked the right projection, it should fit perfectly.
Look for more detailed instructions on using the Georeferencer tool in Chapter 6, “MAP Locations and Data Creation,” of the MAPublisher User Guide.
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