The Magic Bullet
October 1, 2009
As the U.S. Geological Survey’s National Elevation Dataset (NED) moves the nation toward a single, seamless set of topographic data, the recently completed $5.5 million Ohio Statewide Imagery Program (OSIP) provides a blueprint for using LiDAR to create highly accurate, affordable digital elevation models (DEMs).
Over the past three years, every square mile of the state was flown to capture consistent, seamless aerial imagery. But the images would have been just pretty pictures without orthorectification, which is usually completed with traditional surveying and photogrammetric methods. The addition of LiDAR to the imagery program provided a better orthorectification solution as well as a means to meet Ohio’s other geospatial needs.
Data MattersAccurate imagery and elevation data serve as the backbone for development of additional data sets maintained and accessed by government decision makers and the public.
Before OSIP began, the majority of Ohio counties were using older aerial imagery as a point of beginning for myriad projects. But resolution, age and accuracy varied from county to county. The DEMs used to orthorectify the imagery also varied. Twenty-seven Ohio counties had created DEMs using photogrammetric processes. Four counties used USGS DEMs as their source. Nine counties relied on older models. Five didn’t know the orthorectification source, and 36 counties didn’t respond when asked. Seven counties had used LiDAR. Little did these seven counties know that, in a way, they were pioneers.
The State of Ohio’s decision to replace its decade-old black-and-white 1-meter aerial photos with high-resolution color digital imagery began in the late 1990s with an effort led by Stu Davis. As chair of the Ohio Geographically Referenced Information Program (OGRIP), Davis championed a team of key local, state and federal agency stakeholders. The idea was that all would benefit from the imagery--even individuals and organizations, both public and private, beyond the original project participants.
Because the images needed to be converted into usable maps, a DEM had to be acquired to support the orthorectification of the new imagery. However, creating a new DEM was not part of the initial project scope. Davis and his team considered the options: They could use the USGS NED, but at 30 meters between points, these DEMs dating back to the 1970s couldn’t render orthophotos at the desired accuracy level. Alternatively, they could compile a more-accurate DEM of the entire state using conventional ground-based surveying and photogrammetric methods, but that would produce a DEM with limited use and would be prohibitive in time and cost.
Neither of these choices was acceptable. Needing both accuracy and affordability, Davis and his team chose a third option proposed by Woolpert Inc., a full-service design, engineering and geospatial firm headquartered in Dayton, Ohio: Use LiDAR to quickly and cost-effectively collect elevation data that would be accurate within 1 to 2 feet in most terrain.
The LiDAR data proved not only to be a means to generate orthophotography but also became an end product in itself with a number of spinoff uses and applications.
The OSIP PlanThe State of Ohio contracted with Woolpert to collect digital aerial imagery/LiDAR and deliver color digital orthophotography, color infrared (CIR) digital orthophotography and a LiDAR DEM of Ohio’s 41,276 square miles. The northern half of the state was flown in 2006 and the southern half in 2007 and 2008. Recession-related budget issues halted the project for several months, but in early 2009, final post processing was completed.
Using the same equipment and processes ensured seamless data consistency and quality. Woolpert first performed a ground control survey of photoidentifiable points. The existing virtual reference station/continuously operating reference station (VRS/CORS) network of base stations administered by the Ohio Department of Transportation was used for both the ground survey and the airborne GPS employed during flight missions. This standardized network allowed the use of real-time kinematic GPS over large geographic areas and provided a cost-effective way to obtain control for the imagery and LiDAR DEM products.
Leica ADS40 Airborne Digital Sensors were used to collect 1-foot color aerial imagery, 3-foot color infrared imagery, and 0.5-foot imagery for counties that chose to “buy up” to a higher resolution. Leica GPro and Applanix software were used for image post processing.
The team used Leica ALS50 Airborne Laser Scanners to acquire the LiDAR data at 7-foot post spacing. For post processing the data, Leica ALS, Terrascan/Terramodeler, MicroStation and ArcGIS were used. Literally billions of points representing 3D location/elevation data on terrain, waterways, roadways, buildings, and even tree cover were collected for OSIP with an average spacing of 2 meters between points, and these points were used to create new DEMs. The new aerial images were draped over the DEMs to orthorectify the images so scale became uniform and the images, in effect, became maps capable of measuring true distances.
Many UsesThe DEM data accurately map the shape of land surface in 3D. The DEMs can also be rotated to different views and even used to create virtual fly-through videos. Because of this versatility, the OSIP deliverables have many engineering uses beyond their primary purpose. Such uses include hydraulic, hydrologic, and terrain modeling; watershed analysis; landslide prediction and erosion control; flood studies and flood-plain mapping; and related emergency planning, to name a few. Future uses include land and impervious surfaces classification.
The data are consistent and seamless across the state, which allows for “big-picture” planning at local, regional and statewide levels. Instead of looking at just one area and potentially applying the wrong solution, an entire system can be examined and the best overall solution prescribed.
What’s more, accuracy is not sacrificed. The Ohio Department of Transportation compared the LiDAR data to a triangular irregular network (TIN) field survey and found the LiDAR data vertically accurate within 0.30 foot of the field survey. The horizontal accuracy is not quite as accurate as the vertical component, but it is still acceptable for pre-engineering purposes. The data are available through several delivery mechanisms, including an online map service.
Authors’ note: Sources of information for this article include presentations by Mark Myers, former director of the USGS, and George Lee, raster theme coordinator for the National Geospatial Programs Office, USGS.
Sidebar: OSIP Facts and Technical Details· OSIP is a partnership between state agencies and the federal government to develop high-resolution imagery and elevation data for the entire state.
· The project area totals 41,276 square miles across 88 counties.
· 51 northern counties were flown in 2006. Orthorectification of these counties was completed in 2006-2007.
· 37 southern counties were flown in 2007-2008. Orthorectification of these counties was completed in 2008-2009.
· Digital aerial imagery/LiDAR, color digital orthophotography, color infrared digital orthophotography, and a LiDAR DEM were generated for the entire land area of Ohio, including: – Statewide 1"=200' scale color digital orthophotography at a 1-foot pixel resolution. – Statewide 1"=1,000' scale color infrared orthophotography at a 3-foot pixel resolution. – Statewide 2.5' DEM derived from LiDAR, provided in ESRI ArcGRID and platform- independent ASCII grid formats and LiDAR point data provided in LAS formats. – Optional products available for participat- ing counties: 1"=100' scale color digital orthophotography at a 0.5-foot pixel resolution, DTM, or 2-foot contours. – The option to upgrade to 1"=100' scale, 0.5-foot resolution color digital orthophotography. Twenty-six of the 88 counties have currently upgraded.
· LiDAR was used exclusively to rectify digital imagery/create DEMs and to support the generation of 2-foot contours meeting U.S. National Map Accuracy Standards.
· Ohio’s existing VRS/CORS network was used for both the ground survey and the airborne GPS employed during flight missions.
· The OSIP data replaces the circa 1994-1998 1-meter black-and-white digital ortho quarter quad imagery, developed through a partnership with USGS, as well as the USGS 30-meter DEM.
· An online status map viewer displays the status of the various OSIP products and provides access to view the imagery at GIServOhio, gis1.oit.ohio.gov/ website/osip/viewer.htm.
Sidebar 2: Beyond Ohio: The National Map and LiDAROhio is not alone in its effort to provide consistent, seamless data. In 2003, USGS began integrating LiDAR into its National Elevation Dataset (NED) as part of a larger goal of creating The National Map.
Based on interagency partnerships and standards, The National Map will improve and deliver topographic information that is consistent and integrated across the nation. The map will feature eight data layers: hydrography, elevation, orthoimagery, geographic names, boundaries (government units), transportation, land cover, and structures. It will be used for improved hurricane/emergency response, homeland security and scientific studies, and it will be available in the public domain.
WEB EXCLUSIVE: LiDAR Data Application Examples
- ODOT engineers have begun using the imagery and LiDAR data for planning roads, bridges and drainage projects. What used to take a couple of days of field data collection and then a few more days of downloading and post-processing data has been reduced to a couple of hours because preliminary engineering based on the LiDAR data can be handled in the office. Additionally, the LiDAR data coupled with various engineering and modeling software packages can be used to determine whether earth needs to be removed or fill added when preparing a new roadbed. The data can be used in hydraulic modeling to determine waterway flow rates and to install properly sized culverts and drainage ditches. Crews at ODOT outposts can use the data to determine potential areas of flooding and what kinds of equipment and personnel may be needed to mitigate a situation, and they also can do a better job of alerting citizens to potential problems.
- ODOT and public safety agencies are using the data for road-crash analyses.
- In joint projects, ODOT and the Ohio Department of Natural Resources (ODNR) are using the LiDAR elevation and terrain data for landslide prediction and to create scenario models for dam failure.
- The Ohio Turnpike is using the statewide data set to create a seamless GIS program at an estimated cost savings of 45 percent and a time savings of eight to 10 months.
- The data are being used by Ohio’s Office of Environmental Services and other state and local agencies and nonprofit organizations to track and learn more about archeological and historical sites such as Indian burial mounds.
- USGS federal partners will use the OSIP data for urban line-of-site models, flood studies, terrain modeling, and production of new topographic maps and integrated framework data. Among the USGS partners at the federal level are the departments of Homeland Security (including FEMA), Defense (including the National Geospatial-Intelligence Agency), Justice, and Agriculture (including the U.S. Forest Service).
- The OSIP high-resolution DEM data will be integrated into the USGS NED, which provides bare-earth elevation data in a seamless raster format for the entire country. This integration will improve the accuracy and currency of NED in Ohio. It is available in standard formats and also comes with metadata-so, before downloading a data set, users will be able to see if the data will meet their needs. Additionally, all of OSIP’s LiDAR point cloud data will be integrated into the USGS Center for LiDAR Information Coordination and Knowledge (CLICK), a national database for the dissemination of LiDAR data for scientific needs.
- Engineers in the USDA Natural Resources Conservation Service are using the LiDAR data for private land management in Ohio. The USDA division, formerly known as the Soil Conservation Service, works with private landowners (primarily farmers and livestock producers) to optimize resources for sustainable food production while minimizing environmental impact. LiDAR imagery is used with GIS layers, such as soil maps and water resource data, to assess risk and plan applicable conservation practices. The LiDAR data are used to generate contour maps for civil design software applications, which are then used for designing manure storage structures, as well as for creating water-management and wetlands-restoration programs.
- Ohio counties have the option to “buy up” the aerial imagery resolution through a cooperative purchase agreement with the State of Ohio. It’s estimated that the agreement will save the participating counties approximately $4.5 million in taxpayer dollars due, in large part, to the economies of scale realized through a statewide program and the fact that the state is responsible for the cost of project administration and quality control. The buy-ups take the imagery from a 1″=200′ scale at a 1-foot pixel resolution to a 1″=100′ scale at a 0.5-foot pixel resolution. Additionally, 2.5-foot DEMs derived from LiDAR data could be upgraded by the addition of breaklines in order to create 2-foot contours. To date, 26 counties have bought up to the 0.5-foot imagery.
- Google Earth has incorporated the OSIP imagery into its products. Type in an Ohio location in Google Earth, and the imagery is typically that provided by OSIP.
WEB EXCLUSIVE: LiDAR OverviewLiDAR technology uses a scanning laser unit mounted in a camera port on an aircraft. As the aircraft flies along a line, the laser unit emits thousands of light pulses in a side-to-side motion perpendicular to the aircraft's direction of flight (see graphic). These pulses “reflect” off of surfaces – including tops of trees, cars, buildings, and, of course, the ground, capturing enormous amounts of high-resolution topographical data in the form of elevation readings.
The time it takes for each pulse to return to the aircraft is recorded along with the position of the sensor and the angle off nadir (the perpendicular point) at which each pulse was emitted. The system uses an integrated GPS system to record position and an Inertial Measurement Unit to record the attitude (roll, pitch and yaw) of the sensor.
Using this information, one can accurately determine the position where each pulse intersects the surface of the earth or surface feature.
“First returns” provide digital surface models (DSMs)-roof tops, tree tops, open earth. “Last returns” provide the capability to create digital elevation models (DEMs) of the “bare earth” if fully processed to remove buildings, vegetation and other surface features.
LiDAR data can be captured at night as well as during the daytime.