In an earlier blog post, King County GIS Center Manager George Horning wrote about the latest in a more-than-decade-long series of acquisitions of aerial imagery for King County, the 2019 Aerials Project which had reached a milestone when the contracted aerial imagery services vendor, EagleView Technologies, completed their work.
As King County was taking delivery of the full set of imagery, GIS analysts, cartographers, and software engineers across county government, plus users of our GIS products throughout the area, anxiously awaited the creation, from the delivered data, of a variety of secondary imagery products for use in their analyses, maps, and applications.
Now that these secondary products are done and in use, let’s look at the steps that lead to the creation of a detailed, versatile, and well-coordinated set of aerial imagery products.
Preparation for aerial photo acquisition
A goal of an aerial imagery acquisition project for GIS use is to capture and produce a contiguous set of digital imagery that is orthogonal in aspect, that is, it provides a straight-down view of every part of the area to be photographed: the “Area of Interest” or AOI. Further, the imagery should be planimetrically correct, which means the horizontal positions of objects on the ground as measured in the imagery correspond systematically and precisely to their actual positions in the real world.
For each imagery acquisition project, King County defines an AOI that will meet its needs, and specifies the required resolution of the imagery. Due to the clear divide between developed and undeveloped lands in King County—generally between west and east—two resolutions were specified: the more-detailed 3-inches-per-pixel for the western, developed, portion of the AOI, and the less-detailed 9-inches-per-pixel for the eastern portion. Exceptions to this general division are a few eastern locales which are moderately developed, such as a stretch along U.S. Highway 2, including the city of Skykomish, in northeastern King County, and the vicinity of Snoqualmie Pass along Interstate 90. These areas were photographed at the more-detailed resolution.
An aerial imagery vendor plans its flight lines to provide full coverage for the AOI, but King County supplies the vendor with several items which contribute to the vendor’s planning and to its post-capture image processing. One is a digital set of point locations which represent surveyed or otherwise position-established points on the ground, “control” points, that will be visible in the imagery. The control points can be used to help orthorectify the imagery, that is, to adjust it during post-capture processing by stretching, warping, etc., thus offsetting camera parallax and other distortion factors so that the result is planimetrically correct. The vendor typically establishes additional control points to meet its specific flight-planning and image-capture/processing needs.
King County also provides a digital elevation model which the vendor uses in the orthorectification process to make allowances and adjustments for the vertical displacement of ground features. Similarly, 3-dimensional location data for a variety of structures, such as bridges, assists in the correct positioning of those particular structures when orthorectifying the imagery.
Creation of in-house aerial photo products for GIS specialists
The completion of a full set of captured and rectified digital imagery for the entire AOI (illustration below), results in a foundational product, but that product is not in a format that can be readily consumed by display and mapping applications. After an intermediary vendor, Miller Creek Associates in the case of the 2019 imagey, performed quality-control checks of EagleView’s product, King County staff began their work preparing the imagery for various mapping platforms.
King County received the imagery products from Miller Creek on a hard drive, stored as a set of tiled, full-resolution, uncompressed image files, each of which represents a 7500×7500-foot square on the ground. This specific tile array, which covers the entire AOI, is a standard one that King County has used on previous aerial imagery projects.
Once in King County’s hands, the files for the 7500-foot image tiles were compressed using a lossless technique (i.e., no loss of image information) and stored in the King County GIS data library in the GeoExpress MrSid digital image format. In this format, King County staff can access individual or multiple image tiles and load them into GIS software for visual analysis, analytical geoprocessing, and map creation.
More often than not, analysts and cartographers need to be able to work with map extents far greater than a 7500-foot square. So the next step was to create an additional copy of the full set of image tiles in the Esri file geodatabase format, from which a single mosaic of the entire AOI was generated. The digital storage format for the mosaic is called by its developer, Esri, a “mosaic dataset.” A mosaic dataset is actually a system of high-resolution image tiles, lower-resolution “overview” images, a database for storage, and the technology that manages these items to deliver seamless image views on the fly to the user. The mosaic serves as a single, go-to, 2019 aerial imagery product which King County staff can use efficiently in their GIS software to view, analyze, and map any part of the AOI with ease.
To further assist users, a spatial extents layer was generated to represent the imagery, and image metadata then attached to it, to create a reference layer which King County staff can use, via a custom, in-house software tool, to query and identify specific image files when they are preferred.
Creation of aerial photo web maps
A large part of what King County GIS provides to county staff and to residents consists of a broad range of publicly available web mapping tools and products, such as iMap, Parcel Viewer, My Commute, and the ArcGIS Online web mapping system. Zoomable, panable, multi-resolution basemaps are at the heart of the web-mapping experience. And of those basemaps, the aerial imagery ones are very popular and often essential.
So, the countywide 2019 aerial imagery acquisition project wasn’t truly complete until the mosaic dataset was “published” as a web-map service. Compared to the more complex creation of the mosaic, publishing the mosaic as a basemap web service using Esri’s ArcGIS Server technology is pretty straightforward. Similar to the defined, standard 7500-foot array used for image data delivery and storage, King County uses a predefined, standard “tiling scheme” to automatically slice the image mosaic into many thousands of easy-to-download 256×256-pixel image tiles at a range of fixed scales (zoom levels) and stores them in yet another file format which is optimized for delivery of the image tiles on-demand to web browsers as a component of interactive map displays.
That gives us a 2019 aerial imagery product that anyone with a web browser can use! And it gives our aerial imagery planning and production team* a respite until it’s time to prepare for the next aerial imagery acquisition project.
*King County GIS Project Management: Mike Leathers and George Horning; In-house Data Management, Image Processing, and Mosaic Creation: Victor High; Web Basemap Creation and Implementation: Patrick Jankanish and Michael Jenkins.
The final illustration here is the iMap basemap gallery from which users can choose any one of the 20 custom King County basemaps, plus basemaps provided by Esri (not shown). To inspect and compare all of these basemaps, try out our King County Web Basemaps viewing application.