Mapping the History of Southern California Agriculture

Getting Started:

This tutorial will walk through visualizing this database and making it interactive as a web map and public history resource. This map is a proof of concept as to how to digitize research findings and present them to an end-user legibly. This map demonstrates how researchers can add subsequent layers, indicating specific agricultural practices by rancho as portrayed in the diseños. For this tutorial, we will focus on the diseño presentation and relating them to present-day map features.

This tutorial demonstrates constructing a digital map, adding a base layer of a present-day map, georeferencing existing historical maps, and adding photo features. The skills demonstrated in this tutorial can be useful in creating maps for presentations, sharing findings, or simply fortifying a historical map for independent research.

QGIS is a geographic information system software that allows users to analyze and edit spatial information and compose and share maps of raster and vector layers. Our map will show how the software stores vector data as either point, line, or polygon features, and users can modify its information through attribute tables. QGIS also supports a range of raster information and export options. It is open-source software that you can download from QGIS.org.

Base Map:

This tutorial utilizes a base map provided by Open Street Map. This base map will be comparable to a google maps street map; however, some options allow you to decide between what features you would like on your base map and represent them best. This map will be the foundation for understanding our map projections and diseño orientation.

Once loaded, you will have a map projection of our globe in the WGS 84 / Pseudo-Mercator projection. The map should look familiar as it is the same used by Google Maps, Bing, and ESRI. This map projection uses a spherical development of ellipsoidal coordinates. A map projection is one way to represent spherical data as a flat layer, Our map projection has flattened the globe and is a convention for organizing information effectively and comparing it to other digital layers and maps.

We are using a large-scale Union Pacific map of greater Los Angeles published circa 1900, available for download online as a jpg file.

Now we return to QGIS to georeference the portion of the map that we are interested in mapping. We could georeference a more extensive area if the resolution of our downloaded map were higher; further research can achieve this by rescanning a portion of larger maps from archives or locating a higher resolution map to begin the process. Eventually, as the project continues, researchers will digitize the boundaries projected on this map with more specificity. We are georeferencing this layer as primarily a reference so that subsequent digitizing efforts will avoid errors in feature representation.

Georeferencing:

Continue to identify common points between both maps with ground control points. I found the intersection of boundaries to be the most useful. Start from the extent of the area of interest, for instance the corners of rancho la Puente, Merced, and Santa Anita. You need a minimum of six points to georeference your raster. However, more points help your map layers align better. Once you have added at least six points, click the start georeferencing icon, calling up a transformation settings menu.

As you can see, the map projection is not a perfect fit. Especially at the map corners, distortion occurs as the map tries to conform the georeferenced map to the ground control points. This projection is most useful in the specific targeted area, but take your time and add new ground control points, especially at the edges of your georeferenced map. Repeat the previous steps but make sure that the new layer is visible in the layer window and the old layer is not.

Originally I intended to georeference the images in the diseños and maps themselves, forming a patchwork of boundaries and maps. A collage of diseños sounds attractive but took away from the map’s readability

Digitizing:

As you can see, we can take more information from these smaller maps. Still, they are less functional for displaying data over a larger scale due to distortion and limited resolution.

A digital layer is helpful as a guide for digitizing the boundaries of the rancho. Add a new shapefile layer by clicking on the layer menu option, roll-over "create layer," and click "new shapefile layer" to begin digitizing the rancho boundaries.

Note the difference between the projection guide and our more precise digital layer. As we add more information to this map and others, we can use these digital layers as georeferenced guides and populate them with more information, such as the diseño maps.

Adding Photos:

Four of the provided photos contained coordinate locations that I assigned earlier from a free website; however, the remaining three photos in the folder are not displayed by the plugin on the map.

Right-click on the layer properties for the photo layer and click on the display option. Type in this line <img src="file:///[% Path %]" width="700" height="500"> of html code to show the image on mouse over that the point is referring to and scale that image to an appropriate size for the screen. We are calling that image by the variable path to show all of the photos at all of the points.

Next, we will replace the images of the arrows with the diseño images themselves, oriented in the appropriate direction. For this, we will switch the marker again, selecting the raster image marker and then clicking the data-defined override icon, clicking on the field type string to call on the path variable again as we did in the display earlier. Your markers still maintain the same hover functionality; only now they also display the diseño map and orientation.

Webmap:

This map was exported and hosted on an AWS server and embedded in the USDA grant web page. Unfortunately, the software limitations did not allow the markers to be depicted as rasters or oriented however the map functionality was edited to cite the diseños and link to the images when clicked by further editing the labels and attribute table.

The diseño map high resolution scans are available via the California State Archives