LiDAR First Return Digital Surface Model | ||||||
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| Category | Land | |||||
| Feature Dataset Name | Stand-Alone | |||||
| Item Name | LIDAR_FIRST_RET_SURFACE_2008 | |||||
| Database Type | SDE Image | |||||
| Originator | City of Portland | |||||
| Bureau | Bureau of Planning and Sustainability | |||||
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| Abstract | A 3-foot resolution digital surface model (DSM) of all Portland area first-return (a.k.a. "highest hit") LiDAR point returns collected and processed to date (2004 through 2007). The first-return digital surface model (DSM) is a representation of the earth's surface where all man-made structures and vegetation are included. The first-return DSM was created from a subset of LiDAR returns that are classified as "first-return" (representing the tallest object at any given location). | |||||
| Purpose | For creating representations of elevation, slope, and feature height | |||||
| Supplemental Information | source Information LiDAR data
spatial reference info
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| Maintenance/Update Frequency | None planned | |||||
| Extent | Urban Clackamas County, Multnomah County, and Washington County, Oregon | |||||
| Data Type | Raster | |||||
| Shape Type | NA | |||||
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| Raster Object Type | Pixel | |||||
| Row Count | 38032 | |||||
| Column Count | 55254 | |||||
| Horizontal Position Accuracy | 2004/2005 data Vertical Accuracy Vertical accuracy of bare-earth LiDAR data is typically 15 to 20cm (~6 inches). Bare-earth vertical accuracy typically decreases as vegetation becomes more dense, the terrain becomes steeper, or impermeable objects (such as buildings) prevent returns from reaching the surface. A study of the vertical accuracy of the Puget Sound LiDAR Consortium data was performed in May of 2006. This study found an average maximum vertical error of approximately 40cm, with a RMS error of 16.71cm. The complete study can be found online at: http://pugetsoundlidar.ess.washington.edu/lidardata/QA/lowercolumbia05/gcp_results/gcp_lowercolumbia... Puget Sound Lidar Consortium evaluates vertical accuracy with two measures: internal consistency and conformance with independent ground control points. Internal Consistency Data are split into swaths (separate flightlines), a separate surface is constructed for each flightline, and where surfaces overlap one is subtracted from another. Where both surfaces are planar, this produces a robust measure of the repeatability, or internal consistency, of the survey. The average error calculated by this means, robustly determined from a very large sample, should be a lower bound on the true error of the survey as it doesn't include errors deriving from a number of sources including: 1) inaccurately located base station(s), 2) long-period GPS error, 3) errors in classification of points as ground and not-ground (post-processing), 4) some errors related to interpolation from scattered points to a continous surface (surface generation). Conformance with independent ground control points Bare-earth surface models are compared to independently-surveyed ground control points (GCPs) where such GCPs are available. The purpose of the ground control evaluation is to assess that the bare earth DEMs meet the vertical accuracy specification in the PSLC contract with TerraPoint: "The accuracy specification in the contract between the Puget Sound LiDAR Consortium and TerraPoint is based on a required Root Mean Square Error (RMSE) 'Bare Earth' vertical accuracy of 30 cm for flat areas in the complete data set. This is the required result if all data points in flat areas were evaluated. Because only a small sample of points is evaluated, the required RMSE for the sample set is adjusted downward per the following equation from the FEMA LiDAR specification (adjusted from the 15 cm RMSE in the FEMA specification to 30 cm to accommodate the dense vegetation cover in the Pacific Northwest)." During this step, the bare earth DEMs were compared with existing survey benchmarks. The differences between the LiDAR bare earth DEMs and the survey points are calculated and the final results are first summarized in a graph that illustrates how the dataset behaves as whole. The graph illustrates how close the DEM elevation values were to the ground control points. The individual results were aggregated and used in the RMSE calculations. The results of the RMSE calculations are the measure that makes the data acceptable for this particular specification in the contract. 2007 data Real-time kinematic (RTK) surveys were conducted in multiple locations throughout the study area by the vendor (Watershed Sciences) for quality assurance purposes. The accuracy of the LiDAR data is described as standard deviations of divergence (sigma ~ s) from RTK ground survey points and root mean square error (RMSE) which considers bias (upward or downward). These statistics are calculated cumulatively. For the 2007 study areas, the data have the following accuracy statistics:• RMSE of 0.11 feet
2010 data No specific horizontal accuracy information provided. Same vendor and specs as the 2007 data. | |||||
| Horizontal Position Accuracy Link | ||||||
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| Theme Keyword(s) | Elevation, LiDAR, Feature Height | |||||
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| Place Keyword(s) | Portland, Multnomah County, Washington County, Clackamas County, Oregon | |||||
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| Access Constraints | Available for Public Use | |||||
| Use Constraints | These data are distributed under the terms of the City of Portland Data Distribution Policy. Care was taken in the creation of this data but it is provided "as is". The City of Portland cannot accept any responsibility for errors, omissions, or positional accuracy. | |||||
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| Output Geodatabase | EGH_Raster | |||||
| Distribution | Grid: State plane coordinate system 1983(91). Units: International feet Datum: North American Datum of 1983/1991 (HPGN) Liability: The information in this file was derived from digital databases on the City of Portland GIS. Care was taken in the creation of this file. The City cannot accept any responsibility for errors, omissions, or positional accuracy. There are no warranties, expressed or implied. Format: Shapefile or File Geodatabase Feature Class - Data will be provided via City of Portland FTP Site. Online Resource: http://portlandmaps.com/opendata Online Instructions: City of Portland Public/Open Data is distributed via the PortlandMaps Open Data Site - http://portlandmaps.com/opendata. Data not available on the PortlandMaps Open Data site can be requested by contacting the City of Portland CGIS Group - maps@portlandoregon.gov Transfer Size: varies SPCS Zone Identifier: 5076 | |||||
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| 01/01/2004 | LiDAR first return point returns for Tryon Creek watershed. LIDAR missions flown by TerraPoint and pre-processed by USGS. Refer to the PSLC website for more information (http://pugetsoundlidar.ess.washington.edu/). This was a pilot project covering only the Tryon Creek watershed in Portland. Flown in March (leaf off). | |||||
| 01/01/2005 | LiDAR first return points for Portland's West Hills and Columbia Slough watershed. LIDAR missions flown by TerraPoint and pre-processed by USGS. Refer to the PSLC website for more information (http://pugetsoundlidar.ess.washington.edu/). This project covered all of Portland's West Hills and Columbia Slough watershed. Flown in March (leaf-off). | |||||
| 01/01/2006 | 2004/2005 data processing Data was received from PSLC in the form of space-delimited ASCII files in the following format - X coordinate {space} Y coordinate {space} Z Value. Files were organized by USGS quadrangle (i.e., Q45122D64). The ASCII files were automatically read and converted to GRID format using an AML designed for use in ArcGIS Workstation. The AML first converted each text file to a point coverage. A tile system based on quartersections was created, and the points falling within each tile were then used to create a triangulated irregular network (TIN) surface using the ArcInfo CREATETIN command. Where points were within 2' of one another - the "proximal tolerance" of the TIN model - only one (arbitrarily selected) point was used to generate the TIN surface. The TIN was then converted to a 3'X3' resolution GRID using the TINLATTICE command. Finally, the GRID was reprojected to the Stateplane, NAD 83/91 (a.k.a. HARN or HPGN) projection system using a nearest neighbor interpolation method. For more information or a copy of the processing script, please contact the City of Portland, Bureau of Planning (see contact information). For purposes of distribution and ease of viewing, the final GRID tiles were mosaiced into a single image in ERDAS Imagine format. | |||||
| 01/01/2007 | LIDAR first return points for the Metro tri-county region. LIDAR missions flown by Watershed Sciences and pre-processed by USGS. Refer to the PSLC website (http://pugetsoundlidar.ess.washington.edu/) or the Oregon LiDAR Consortium website (http://www.oregongeology.com/sub/projects/olc/default.htm) for more information. This project covered all of Metro urban growth boundary area, Bull Run watershed, Mt. Hood, and areas around Hood River. The Portland portion of the study area was flown between March 15 and April 15th of 2007. Complete documentation of the data collection and processing is available upon request. | |||||
| 09/01/2008 | 2007 data processing Data were received from the vendor in the form of raw LAS files (organized by quarter-quad tiles) and 3' resolution DEM images in GRID format (organized by quarter-quad). The GRID files were mosaiced into a single image, which was then mosaiced with the 2004/2005 LiDAR DEM ERDAS Imagine image. Only those tiles intersecting one of the 5 major watersheds within the City of Portland were included in the final mosaiced image (Columbia Slough, Fanno Creek, Tryon Creek, Johnson Creek, and the portion of the Willamette Basin within the City limits). All work was performed in ERDAS Imagine and ArcGIS. | |||||