Several years ago as a new project lead, I was put in charge of a project for an Oregon county in which my team was to create one of the first operational ORMAP geodatabases in the state. It was a time of transition from the older ArcInfo coverage files to the newer, shinier, silver graphic cylinder of the geodatabase. There was a considerable amount of buzz around this new transition and how this would increase the accessibility of GIS data to a web-based platform for all to marvel. While this promised much, there was still a few hiccups in the workflow – namely, GIS software at that time still did not have robust coordinate geometry (COGO) tools available and once the geodatabase began to fill, the software became sluggish.
The team was soon longing for the sweet comfort of MicroStation (V7) to quickly draft the cadastral boundaries with a relatively painless conversion using arc macro language (AML) to coverage format. Good for drafters, but the limited final product of linework with minimal intelligence was the writing on the wall that the days of the coverage file were numbered and that the geodatabase would be here to stay. We would all just have to endure the somewhat tedious growing pains of the software, knowing that in the end, the wait would eventually pay off.
We’ve collectively come a long way since then in the mapping industry, but in my experience, it is often necessary to blend older technology with the newer. This can be to transcend project budget limitations or reflect new, dynamic and changing conditions of infrastructure with pre-existing data; even when that data originates from the latest and greatest technology or software.
Such was the case in one of our more recent projects. Our photogrammetry team lead by Brad Hille, was tasked to deliver seamless section tiled orthophoto mosaics developed from over 1,005 individual digital orthorectified images, each at a resolution better than 6 pixels for a Public Utilities District (P.U.D) in the Northwest. Existing, but outdated LiDAR data was provided by our client and while very cutting edge technology, regional terrain had changed since that data acquisition.
In this project, we checked relative accuracies of the existing LiDAR DEM (Digital Elevation Model) against the new aerotriangulated stereo imagery, then collected supplemental DTM (Digital Terrain Model) data within the necessary update areas. The result was the use of existing digital terrain data with limited update supplementation that yielded an accurate, high quality, updated orthorectified imagery coverage throughout the Public Utility District.
The project began with aerial imagery acquisition from a photogrammetric flight and ground control plan designed to provide the P.U.D. with the best quality and optimum coverage for their project area. i-TEN Associates had the project flown at 5.5 GSD (Ground Sample Distance) to ensure that 100% of the imagery would be of greater detail than the required 6 pixel resolution; regardless of terrain or elevation changes in the aircraft. An Aerial Triangulation (AT) solution was generated with the imagery and technicians commenced with verifying and updating the Digital Terrain Surface Model, highly accurate positioning collection of utilities and other features within the district, as well as production of the orthorectified imagery.
Creating an accurate and current Digital Terrain Surface Model, allowed i-TEN to provide the P.U.D. with an engineering quality orthorectified image, as well as a precision planimetric base map that replaced the unreliable and inaccurate existing data. i-TEN having already analyzed all of the existing DTM/DEM data available, had found that about 55% of the P.U.D. was covered by sufficiently accurate existing LiDAR data.
DTM & TIN – Filling in the blanks with proven methods
Those areas found to have had significant changes between existing conditions and outdated LiDAR data, were augmented by precision breaklines and mass-points, generated in house by our expert photogrammetry staff and i-TENs proven methodologies. Additionally, i-TEN augmented the existing and contemporary USGS LiDAR DEM.
From mass-points and breaklines, i-TEN generated a TIN, which represents the terrain surface as a set of contiguous, non-overlapping triangles. Creation of the TIN surface served to verify that the collected DEM accurately captured the characteristics of the terrain surface. This methodology resulted in the most reliable and accurate source of terrain data to support the image rectifications and the accurate planimetric mapping of the service area. The final orthophoto generation and deliverables included one set of digital color balanced orthophotography with a 6inch pixel resolution for the defined service boundary. All project areas included 4color band (red, green, blue and NIR) imagery. A representation of our final product is shown below.
The leveraging of older mapping methods with LiDAR data is a relatively common practice in the photogrammetry world today. While LiDAR as a fundamental data type is incredibly reliable, there will be cases where alternative mapping methods drawn from years of experience can ensure contemporary maps are up-to-date with high accuracy regardless of nature’s constantly shifting landscape.
Tools of our Trade
i-TEN utilizes the new 4 band (including false color I/R) Vexcel UltraCamXp digital camera for flights. Our photogrammetry team uses industry standard photogrammetry software which includes the Intergraph ImageStation Photogrammetry Suite, Intergraph ImageStation Automatic Triangulation, ImageStation DTM Que, Intergraph OrthoPro, and DAT/EM Summit Evolution.