Overview of Steps

Note

These steps are best carried out on a laptop/desktop computer, though a nimble mobile user should be able to perform them as well.

In this application, the entire georeferencing process comprises three sequential steps, each with its own tool interface:

  1. Preparation (Splitting)
  2. Georeferencing
  3. Trimming (optional)

These steps need to be completed in order, but they don't have to be completed all in one sitting. In fact, Georeferencing and Trimming can be revisited at any time, so you can improve on your past work, or build from someone else's.

Tips for using each tool accompany its interface, and similar content is presented below as well.

When you are ready, you can create a web map from layers that have been georeferenced.

Preparation (Splitting)

=== "Basic concept" Before a Document can be georeferenced, it must be visually evaluated to determine whether it contains more than one part of town in it. If it does, each of these parts must be split into separate Documents.

=== "Tell me more..." When an image is georeferenced, as you'll see below, control points are made that link pixel coordinates on the image with latitude/longitude coordinates on the earth. This means that a set of control points must only be linked to a specific geographic area, i.e. a single file.

The splitting interface, ready to split this document into two new documents.

Background

Sometimes an old map document will cover discontiguous areas, especially when the mapmakers were trying to fit a lot of content into a single page. In these cases, each separate area in the original document must be split into its own new document, so that each area can be georeferenced on its own. Typically, you'll find strong black lines delineating different parts of the map. The document must be split along those lines.

This map must be split into four new documents.
This map must be split into two new documents.
This map shows only one part of town, so it should not be split.

Using the Interface

If this document does not to be split:

If this document does need to be split

Fixing an incorrect determination:

Creating cut-lines:

Understanding cut-lines:

Georeferencing

=== "Basic concept" "Georeferencing" is the process that is needed to overlay a scanned historical map onto a modern web map, and it must be performed for each Document individually.

=== "Tell me more..." Georeferencing works by using "ground control points" to embed geospatial information into an image file and turn it into a geosptial dataset. A ground control point consists of two coordinate pairs: one pair that represents the XY pixel location on the document, and a corresponding latitude/longitude coordinate that represents a point on earth.

The georeferencing interface, with 3 control points added and the preview visible.

Background

Use this interface to create the ground control points that will be used to georeference this document. C├čreating a ground control point requires two clicks—once in the left panel and once in the right. This records a linkage between a spot on the original map document and the real-world latitude/longitude coordinates for that location.

Once 3 control points are present, a semi-transparent preview will appear.

In the example image above, 3 control points have been made using street intersections. You can make as many control points as you want (the more the better!) but often 3-6 are enough. If 3 or more are present, a semi-transparent live preview will be added to the right panel. Use the w key to toggle preview transparency.

Using the Interface

Before starting, it can be helpful to pan and zoom around to become familiar with the document and the area.

Creating a control point:

Deleting a control point:

Saving Control Points:

Editing existing control points:

Managing layers:

Managing the panels:

Transformations:

Tips:

Trimming

=== "Basic concept" You can trim the edges of a border layer away so that adjacent sheets on the same web map don't obscure each other.

=== "Tell me more..." This process is accomplished by creating a polygon "mask" that is used to crop extraneous layer content. These mask coordinates are written into an alternate layer style that is set as the new default for the Layer. This approach preserves the original style, which allows users switch back to the full style if they want to see the entire image in a web map.

The trimming interface with a mask trimming (slightly) the edges of this layer.

Background

To use our georeferenced layers most effectively, we can combine them in web maps to create a seamless mosaic. However, this often causes adjacent images to overlap and obscure each other. Consider the example below, where the layer on the left covers up the edge of the layer on the right.

Two georeferenced layers are shown on top of an aerial imagery basemap. The left layer overlaps the right layer.

To reduce this overlap, we can trim the edges of the left layer by creating a mask. This is just a shape that will cause everything outside of it to be clipped away. The result can look like this:

The left layer has been trimmed to create a seamless mosaic.

The mask polygon is stored as a new trim style for the layer, and set as the default style. The layer itself is not changed, however, so don't worry about altering any underlying data through this process.

Using the Interface

Creating a new mask:

Adjusting the mask:

Altering an existing mask:

Tips:

Creating Web Maps

=== "Basic concept" Users can author their own Web Maps, and add whichever layers they want to them. This is most obviously useful for aggregating all of the layers for a single city in a single year, but you could also combine layers from different years.

=== "Tell me more..." Creating Web Maps is a core GeoNode functionality, and this project only scratches the surface of what these maps can do. Please see the GeoNode documentation to learn more.
Please note: In GeoNode parlance, what we refer to here as Web Maps are simply called Maps.

The easiest way to create a web map is to start in the main layers search page. Here you can use some search criteria to find the layers you are interested in, select all of the layers you want with the button, and then click Create a Map.

With one or more layers selected, you can create a new web map from this interface.

This web map will be visible to anyone and embedded into other websites, and if you view it on a mobile device you can add your current location.

Adding Layers to an Existing Web Map

You can also add layers to an existing web map. In the top right corner, you'll find the Catalog button. Use this list to find and add other layers.

Adjust layer order in the left-hand panel, change basemaps in the bottom left corner, and save options on the top right corner.

If you are creating a map to cover an entire large city, this strategy will likely be your only course of action.