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Mapping Class: Georeferencing Techniques Part One – The Basics, with Hans van der Maarel

Welcome back to another exciting edition of Mapping Class, a new video-blog series where we curate tutorials and workflows created by expert cartographers and Avenza power users from around the world. For this article, we are excited to introduce Hans van der Maarel, owner of Red Geographics, and expert cartographer. Joining us from Netherlands, Hans has put together a video tutorial showcasing tips and tricks for tackling Georeferencing in a variety of different mapping scenarios. In this first part, Hans goes over the basics of georeferencing in MAPublisher, using a neat city map of Zevenbergen. Tune in for Part Two, coming soon, which will reveal how Hans approaches more challenging georeferencing tasks, including dealing with unknown projection information and working with historical maps.

Hans has produced a short video walkthrough detailing part one of his georeferencing process. The Avenza team has produced video notes (below) to help you follow along.

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Georeferencing Techniques Part One: The Basics
by Hans van der Maarel (video notes by the Avenza team)

Georeferencing is the process of taking imagery or map data that lacks geographic location information and associating it with specific coordinates on Earth. Georeferencing is a very common, but sometimes challenging step that is necessary for producing accurate, meaningful cartographic products. By georeferencing map data, cartographers can ensure that the features on their maps are located correctly, and in a way that accurately represents the real world. Georeferencing also makes it easy to add and update maps with new data layers, as location information stored within the new map layers will be accurately overlaid in the correct position on older map projects. The process for georeferencing maps can be complicated, but Hans has outlined some easy-to-follow steps for quickly performing and validating simple georeferencing tasks with vector map data.

In general, effective georeferencing needs to include at minimum three known control points. In this example, Hans has included an additional fourth control point to provide additional accuracy. 

When locating control points, it is a good idea to choose points that roughly approximate the four corners (quadrants) of your map area. Doing so can ensure the georeferencing result is accurate for the entire coverage of the map area and minimizes distortion/shearing effects as the map layers are matched to the final coordinate system. Cartographers should take time to ensure the chosen control points are as accurate as possible, as errors in control point placement will propagate across all locations in the map. Poor control point placement can lead to overall poor georeferencing accuracy. 

Using the MAP Page location tool, place four control points at known, easily identifiable locations. Hans recommends placing control points at recognizable map features that can be easily seen on the reference imagery. For this example, Hans chose to use the corners and edges of major structures (i.e larger buildings/reservoirs) or the centers of well-known major road intersections. When using road features as reference control points, Hans recommends using the center of the feature rather than the edge. This can compensate for variation in road edge placement that can occur when the vector line layer does not completely match the true road/lane width in the imagery.

Mapping Class Georeferencing control point

Next, open the Georeferencing tool and select the “Add World Locations” option. From here, use the built-in web map to calculate latitude/longitude coordinates for each of your known control points. Using the satellite imagery view can make this process easier, especially when dealing with physical features on the map (i.e building corners). Repeat this for each of the four control points.

The resulting table will show a list of set coordinates for each of these control points. From here, if you already know the projection the map data is already in, you may set this coordinate system at this stage. If you are unsure, the georeferencer tool will automatically provide a suggested list of coordinate systems that match the control points you have set. These “best” matches are provided based on measuring the error between your user set coordinates and the real-world locations on the web map. Ideally, you want the lowest combined error value. In general, the suggested coordinate systems at the top of the list are often the best choice.

Once you select the desired coordinate system, the tool will automatically create a new MAP View where you can house your newly georeferenced map data. You will notice that the MAP Page Locations you created earlier will be displayed alongside the newly georeference control points. This is a great way to help validate your georeferencing as you will be able to observe the accuracy (or inaccuracy) of your placed control points.

Finally, it is a good idea to use the Find Places tool to validate your georeferencing results. Try searching for identifiable landmarks or major features on your map (i.e. train stations). Simply search for a location using the Find Places tool, and compare this to the georeferenced locations on your map.

This concludes Part One of “Georeferencing Techniques with Hans van der Maarel“. Now that you have covered the basics of Georeferencing in MAPublisher, tune in for part two in the next edition of Mapping Class. There you will see how Hans tackles more complex georeferencing projects, including what to do when you have small-scale maps that come from scanned or printed images, or where projection or referencing information is unavailable. Hans will be using a beautiful historical map of northwest Africa to demonstrate this problem. Look for it in the Avenza Resources Blog next month.

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About the Author

Hans van der Maarel is the owner of Red Geographics, located in Zevenbergen, Netherlands. Red Geographics is a long-time partner of Avenza and Hans is a well-known power user of both MAPublisher and Geographic Imager. He uses the products for a wide range of cartographic projects for several international organizations and offers training courses and consultancy expertise aimed at developing workflows for clients. In addition to that, he is currently a board member of NACIS. To find out more about Red Geographics, and to see more work by Hans, visit redgeographics.com

MAPublisher Features We Love

MAPublisher has been simplifying the process of making maps beautiful for cartographers for more than 20 years. We are always adding new features and improving others, some of which have impacted the overall workflow and affect a majority of users. Other are more ‘niche’ in their application and the functions they perform. Here are a few favourite features that you may, or may not be aware of, as identified by the people who helped design and build them.

MAP Tagger Tool Michael L. – Product Marketing
I like the MAP Tagger Tool because it’s incredibly fast to create labels by clicking features on the artboard. Labels are created using attribute data as a source for the labels. In dense areas, the Map Tagger has flexibility to style and attach leader lines according to placement rules.

MAP Web Author Will H. – Sales
The MAPublisher users I speak with are usually impressed with Web Author and are surprised that it is included in the MAPublisher package. MAP Web Author lets Adobe Illustrator documents with GIS attributes be exported to interactive HTML5 web maps complete with callouts, rollovers, layer control, search, pan and zoom controls. With a little bit of knowledge of CSS (Cascading Style Sheets) and JavaScript you can embed interactive maps into any web page. The map below was created by the National Park Service.

MAPublisher MAP Web Author

Scale and Rotate by Attribute Andrew P. – Software Architect
We added this for a customer who had an interesting use case involving a pattern fill for lava flow. The map broke down an area into sub-areas by lava flow (direction, intensity, etc.). He wanted a way to use the attribute data he had in his map to automatically do a bunch of things he would otherwise have to do by hand, very carefully. In particular, he used the feature’s ability to rotate the pattern of area to match the lava flow direction, which sounded very cool (no pun intended) to us!

MAP Locations Tool Michael L. – Product Marketing
Most users don’t know what MAP Locations does (allows you to define real world coordinates for a location in a document) and how it’s actually useful. It sounds complicated, but it’s actually simple and far reaching throughout the product. MAP Locations can be used in several MAPublisher tools as references for georeferencing, for corners, for locations to draw lines, and locations to plot points. They can also be used to identify map and page anchors.

MAPublisher MAP Locations Tool

Add Calculated Data Andrew P. – Software Architect
Add Calculated Data is essentially a tool to update or add attribute data, but it allows users to feed in things that a user would find difficult to calculate themselves such as centroids, north angles, and art bounds. It also lets you pull in and store data like stroke or fill colours, in case you need to export them to a format that doesn’t support colours natively. You can even have it pull in elevation data!

The Avenza Resources Blog regularly published tips on how to use various MAPublisher tools. Detailed documentation is also available on our website.