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.

• Historic maps - what used to be here?
• Photos of non-current political district maps...snap a photo of that old map you see in a public building
• Maps from community development corporations, from old hand drawn annual reports and the like
• Zoning maps from small municipalities that don’t have a GIS department
• Community plan drafts handed out at public meetings (likely created in CAD/GIS, but without easy access to the consultant, etc.)

What are the possibilities of using georeferenced maps? Why take the time?

First, Let's find some images of maps!

Important!

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: digitalcollections.nypl.org

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 buildinginspector.nypl.org and spacetime.nypl.org for some interesting historic mapping projects from the NYPL.

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

RASTER

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:

• satellite imagery

• weather data

• digital elevation model

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

mapschool.io

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

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

Georeferencing

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

basic steps

to georeference

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.

Rectify

Save the image to coordinate space

Technically | source

stretch that map | source

What we will do

1. Familiarize with QGIS
2. Adding Basemaps with OpenLayers Plugin
3. Using the Georeferencer
4. Geotiffs and where we can use them
5. Generating Raster Tiles with the QTiles Plugin
6. If there is time, Uploading Raster Tiles to Github

QGIS Interface (QGIS Training Manual)

1. Layers List | 2. Toolbars | 3. Map Canvas

4. Status Bar | 5. Side Toolbar

Adding Baselayers like OpenStreetMap, Google Maps

with the OpenLayers Plugin

Adding Baselayers like OpenStreetMap, Google Maps

with the OpenLayers Plugin

Installing OpenLayers to your QGIS

1. Select the Plugins Tab, and select Manage and Install Plugins
2. In the search bar of the window, type "Openlayers"
3. At the bottom right of the window select install

Loading basemaps to QGIS

1. Select the Web tab and select "OpenLayers plugin"
2. Select your desired map

Georeferencing a scanned map

1. Let's use start
2. with Union Station

Georeferencing a scanned map

1. Add a basemap to serve as the coordinate space
2. Use OpenLayers and add OpenStreetMap
3. Zoom over to Union Station

Georeferencing a scanned map

1. Select the Raster Tab and select "Georeferencer"

Georeferencing a scanned map

1. Add the raster map of Union Station
2. Because we are going to make webmaps in the Coordinate Window, select WGS84

Georeferencing a scanned map

1. Your map should be loaded into the map window

Georeferencing a scanned map

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

2. Avoid using "soft" references, like shorelines.

Georeferencing a scanned map

1. Let's start with the Post Office, click at the corner to open the map coordinate window.

2. To get the coordinates, select the "From map canvas"

Georeferencing a scanned map

1. In the basemap area, zoom and pan to the corner of the Post Office and click a the corner to obtain the coordinates for your georeferencer

Georeferencing a scanned map

1. Add as many control points as necessary to better rectify the image.

Georeferencing a scanned map

1. Now let start the georeference!

2. Hit the play button on the toolbar, it will ask you for select a transformation type.

Georeferencing a scanned map

1. Basic settings include setting the transformation type to "Thin Plate Spline" to the nearest neighbor

2. Select the "Target SRS" to be WGS84

3. In the Output Raster, select the area where you want QGIS to save your georeferenced image.

4. Set the "Use 0 Transparency" if the image may get rotated.

5. Make sure the "Load in QGIS when done" is set to have the image load on your map in QGIS

Georeferencing a scanned map

1. After you hit OK, you be back to the georeferencer. Unless you have already made the transformation settings before, nothing has happened.

2. To begin georeferencing, hit the green play button.

Georeferencing a scanned map

1. Your image is now georeferenced!

2. You can now add additional layers for your projects or take the geotiff and load it into Google Earth, Mapbox and many other mapping tools.