How to Georectify Historical Maps using MapWarper

Map Warper (https://mapwarper.net/) is an open source map warper / map georectifier, and image georeferencer tool developed, hosted and maintained by Tim Waters.

Creating an Account

While it is possible to browse and download maps others have uploaded into Map Warper without an account, to georectify your own map, you must make an account. This also allows you to easily return to your maps later.

All you need to create an account is an active email address. It may also be linked to an active Facebook or Github account.

  1. On the top right corner of the page, click "Create Account"

  2. Select a username and password and enter an active email address.

  3. Click "Sign up"! You should quickly receive an email to confirm your account

Choose "Create Account"...
...and enter in a username, password, and active email address!

Upload a Map with any Available Metadata

Now that you are logged in, you can upload your own images to the Map Warper server in order to georeference them.

By uploading images to the website, you agree that you have permission to do so, and accept that anyone else can potentially view and use them, including changing control points. As a freely available tool, you should not expect Map Warper to store your map indefinitely; once it has been georeferenced, you should plan on storing your georeferenced map on your local hard drive or a file storage platform like GoogleDrive.

  1. Clicking “Upload Map” on the main toolbar (note if you are not yet logged in, it will ask you to do so at this point)

  2. Insert any available metadata and a description of the map. This is useful both for your own records and for anyone else searching for similar maps on the Map Warper server.

  3. On the bottom of the page, choose to Upload an Image File from your local computer or from a URL. Once the file has been selected, click "Create"

Once you are logged in, select "Upload Map"...
...enter any available metadata...
...and then choose what image you want to georeference, and click "Create"!

Once the upload is complete, a new page will appear informing you that the map was successfully created as well as providing an image of the uploaded map.

Rectify a Map with Control Points

Now the map is on the platform, but it does not yet have any spatial information associated with it. The next step is to use what are called "control points" to place your map in a “real-world” coordinate system where it can interact with other types of spatial data.

Note that you can also edit the original metadata fields, crop out unwanted portions of your map, and see a history of the interactions with the map at this point from the main toolbar

Understanding the Georectifying Window

  1. Once your image is displayed, select "Rectify" on the main toolbar.

  2. This opens up the Georectifying page, the most important page in this tutorial. It is composed of two windows, one showing your map and one showing a “basemap” which you will be using to geolocate your map.

  3. In the top right corner of each map there are a series of buttons that help you navigate the map and add control points

  4. The goal here is to create what are called “control points” or points that are corresponding between your uploaded map and the basemap. This is done by simply zooming in on each map in turn and creating a control point as close to the same point as possible in each map.

Georectifying page, with your map on the left and a basemap on the right...
...and each has a series of icons in the top right corner

Add a control point to the maps

Move a control point that has already been added to the map

Pan around the map

The last two icons appear only on the basemap and are used to adjust it as needed to help with georeferencing

Change the basemap to a Custom, already georectified basemap of your choosing (if available)

Swap between the initial basemap and satellite imagery, depending on which is easier to georeference your map

Creating Control Points on your maps

  1. Navigate on your map to an easily identifiable location. In this example I have chosen the tip of the island in the middle of Paris that the Notre Dame Cathedral is on. Note that an external mouse with a scroll wheel can make the zooming/moving process easier; zoom and pan buttons are also provided in each window.

  2. Click the “Add Control Point” icon, then click again on your map in the desired location. A little control point should pop up!

  3. Swap to the basemap and click the “Pan” tool (the hand) to find the proper location, then again select the “Add Control Point” tool and click on the corresponding point on the Basemap.

  4. Once you have created a control point on each map, scroll down and click the “Add Control Point”. This will add the control point coordinates to a list of points below, which you can see by clicking the words “Control Points”, and give it a number on the map.

Click the Control Point icon then choose a location on your map...
...and repeat this process on the basemap at the same location!
Finally, click "Add Control Point" to set the two points equal to one another.

You will need at least 3 control points to geolocate your map, but more is preferrable. It is also advisable to spread your points across the map rather than have them clustered; this will ensure that the map is georeferenced equally across the map rather than only in one area. If you need to delete a point, this option is available from the "Control Points" table

Remember: places change over time! Try to use features that remain as consistent as possible on both maps. In general, the more control points you add, the more accurate your map will be.

Check for Error and Warp the Map

  1. After you add the 4th control point, your table of points will start including error information, as the points are triangulated against one another. Note that this error may not mean that you are doing anything wrong, particularly in an older map that is not as spatially accurate as something more modern! On the other hand, if your error is quite high and you believe your map is relatively accurate, you may have misplaced a control point somewhere. Usually high error is caused by a single point being misreferenced.

  2. When you feel like you have enough points scattered around your map, we are ready to georectify the map! Remember you can always come back later and add new points or remove old ones if you feel like the result is not to your liking. To georectify your map, just click “Warp Image!” at the bottom of the page and you’ll get a notice that your rectifier is working.

Error is shown in the Control Points toolbar, color-coded by suggested severity.
When you are ready, hit "WARP IMAGE!"

When the map is finished rectifying, you will get a notification that rectification is complete. Now, you should be able to see your map overlaid on the Basemap, as well as be able to turn it on and off or more or less opaque to check for accuracy!

A pretty accurate map of central Paris!

Exporting your Map

If the map is to your liking, you are ready to export. Map Warper offers a variety of ways to export your map depending on your needs

  1. To export your map, Select the Export tab on the toolbar. A window like that seen below will pop up, giving you a variety of choices for exporting

Lots of options for exporting

Some exporting options:

  • GeoTiff:

    • public domain standard; easily imported into a wide variety of spatial platforms like ArcGIS or QGIS; good for backing up your georeferenced map on your local computer or in cloud storage like Google Drive

  • .kml:

    • Easy import directly into GoogleEarth

  • Tiles (Google/OSM scheme)

    • Useful for loading into tools like ArcGIS online and Knightlab StoryMap JS. Remember to ensure a backup of your files elsewhere though in case your map is eventually removed from Map Warper.

Adding your tiles to an ArcGIS online map can be complicated. From an empty map, choose Add --> Add Layer from Web and then select a "Tile Layer". Where it says “URL” copy over the Tiles (Google/OSM scheme) URL from your Map Warper file. It will look something like: https://mapwarper.net/maps/tile/49503/{z}/{x}/{y}.png.

However, note that the end of the URL should look like “{level}/{col}/{row}.jpg” according to the instructions given. Replace the {z}/{x}/{y}.png at the end of your URL with this ending, creating something that looks like: https://mapwarper.net/maps/tile/49503/{level}/{col}/{row}.jpg. It should now load properly into ArcGIS online