Georeferencing Maps with Map Warper

@MaptimePGH | 27 April 2016

Follow along at /pittsburgh /presentations /2016-04-27-mapwarper

Find that link at Look for Map Warper under the MaptimePGH Resources sidebar.

What is Maptime?

Events / blog n'at:


PLEASE interrupt if you have questions!

Today we're going to talk about
Map Warper

and the process of putting images of maps into a digital geographic context:


Why might you want to do this?

You may not have access to the data used to make the map, but want to see the map in the context of other geographic data.

First, Let's find some images of maps!

University of Pittsburgh's Historic Pittsburgh Maps Collection:

Explore their catalog - very cool stuff!

Full-Image and Description

See An Example

Full-Image provides a little image-viewer. Zoom and pan around.

Description provides important metadata: Title, Date, Creator, etc. Also, read the Rights and download a copy.


Pitt's maps themselves (which we will use for our examples tonight), and many historic maps are in the public domain.

However, always be cognizant of copyright and license when using the work of others.

New York Public Library (NYPL) Digital Collection:

NYPL includes some maps outside NYC. Search for “pennsylvania map” and check the “search only public domain” to see some great stuff! (and you can filter down by “place” once you are in the main search area.)

Also see and for some interesting historic mapping projects from the NYPL.

Penn State Digital Sanborn Maps:

Great collection, and Sanborn maps are relatively easy to georeference due to the map projection (a little more on that later).

All the maps maps on these sites are stored as image files.

The files themselves contain no machine-readable geographic information.

If we want to view these maps in a geographic context, with other geographic data...

we need to georeference them

What does it mean to georeference an image?

First, let's talk about the basics of images on computers and maps.


A raster stores data in pixels

On maps, pixels are powerful.

In Geographic Information Systems (GIS) software, the pixels of a raster can represent color, height, slope, direction...and many other classifications or gradients.

Geographically-referenced raster data is commonly encountered in:

Such rasters contain geographic reference information that describes where the pixels exist on the globe.

This info typically includes the coordinate system and some information about the raster's dimensions, in a standard format.

The historic images do not (yet) have this information.

Some references on coordinate systems:

MaptimeHQ: The Wild World of Geodesy

MaptimePGH: Geo101

Raster as a text file (ascii) with geo info:


the process of giving non-geographic data the geographic attributes it needs to be shown in the correct location on a digital map

Read Esri's Fundamentals for georeferencing a raster dataset for a good conceptual overview

Download a map from the University of Pittsburgh's Historic Pittsburgh Maps Collection that you'd like to georeference

Some maps will be easier to georeference than others.

Find one with a lot of detail. For your first map, try to avoid broad-scale or conceptual maps.

Map Warper

"It's a free to use, open source map warper / map georectifier, and image georeferencer tool. Developed, hosted and maintained by Tim Waters. This project is supported by Topomancy LLC and the New York Public Library".


How do I georeference an image using Map Warper?

  1. Sign-Up
  2. Upload map and enter metadata (really, really important!)
  3. Georeference (Rectify, Crop, Align)
  4. View!
  5. Export to a file or reference as a web service
  6. (make fixes)


Make sure to copy metadata from your source!!!

Be thorough! Give credit!

(you can add more later if you need to)

Rectify using Control Points

Image Space,
Coordinate Space

our map image at left,
our web map at right

Identify Control Points

A little bit of art
to the science...

Find clearly-defined points of reference,
like buildings or intersections.

Avoid using "soft" references,
like shorelines.


Save the image to coordinate space



Stretch that map!


Crop and Align

Crop lets you cut off portions of the image. Useful if you scan has extra white space. But please avoid cropping any info that might help identify the map.

Align lets you specify other maps you've uploaded to Map Warper

Preview Map

Now that I've georefernced this map...what do I do with it?


Work Session!

(tag your maps with MaptimePGH)

Share Your Work!

Q/A + Troubleshooting

Did anyone have a map didn't work?


Carnegie Library of Pittsburgh

OpenPGH, Pittsburgh's Code for America Brigade

Maptime HQ and the other Maptime chapters for providing great resource material

Our next event will be announced soon, for end of May / beginning of June.

additional MaptimePGH resources

Web Mapping + CartoDB

Geo 102 & Web Mapping (for the 2015 National Day of Civic Hacking)

Geo 101 & Open Street Map

Maptime HQ resources page

Presentation pulled together by @GassChristianB and @eleanortutt

Presentation made with Stack.

Stack is made by and is available
on GitHub under the BSD license.