The Public Laboratory for Open Technology and Science
(PLOTS) is an organization and membership community which develops and
applies open-source tools to environmental exploration and
investigation. Public Laboratory’s mapping tools, openly available and easy to use,
are putting the ability to do processes such as georectifying in the
hands of people who may have never created a map.
Using aerial mapping techniques, residents and volunteers of the Gulf Coast region began field mapping trips in 2010 to document the impact of the BP oil spill. Between May 2010 and April 2011, tens of thousands of images were collected and 50 regional maps created. Between May and October 2011, Public Laboratory partnered with Dr. Alex Kolker, from the Louisiana Universities Marine Consortium to begin a bi-monthly monitoring of select oil impacted and non-impacted sites in the Barataria Bay region. The intent of this phase of wetlands mapping was to monitor change over time with high-resolution aerial and ground imagery.
MAKING MAPS WITH GEORECTIFICATION
The fieldwork that goes into collecting images is the first step in creating maps. On the back end, the next step involves georectification — the process where balloon and kite aerial images are “published” into geographic data. Simply, it is the alignment of an aerial image with a map or other spatial data of the same area. This part of the process is where images become maps and are associated with geographic standardized formats so that other users and programs can exchange and experience them in the same context.
Public Laboratory map production has used some specific techniques in creating wetlands maps. The maps are made through a georectification process by which adjacent images from the flights are merged in overlay as they are aligned to existing mapping information. Different “base” data can be used in these types of projects. In this case, we’re georectifying the imagery through examination of the new images with existing imagery.
A new unaligned image (right) about to be georectified with the other imagery. The base layer is visible in the background.
Distortion correction is applied to the new images, and they’re moved around so that the same features of the base map are in perfect overlay with the new image on top. In the bayou setting where lots of change is occurring on the outer coastlines, features such as vegetation composition and interior waterways are used to match each overlay. Although waterways in marshland may shift quite rapidly, the center of a 3-way intersection is quite stable, as a rule.
Matching the imagery based on the interior features has proven effective in this simple map-making technique. While the outer coastline in our new images has changed since the time of the base data, the process of fitting the imagery with all of the interior ground control features allows us to discover where the coastline is with some measure of confidence. Or to put it differently — when the alignment happens with the historic data in most of the image, then the new areas can be extrapolated with regularity.
The image is aligned in overlay with the base data during the georectify process.
Visit the Public Lab site for guides and discussion about the process and to view image sets and maps that have already been published in the map archive. The archive is a home and distribution channel for published open-source maps.
Visit MapKnitter.org to learn about our map-stitching tools and to view maps that are being created by people who are using aerial mapping techniques in new ways to document and monitor sites and events that are of importance to their community.
