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Geographic Watershed Information System (GWIS) DEM Modeling and Terrains with ArcGIS

Geographic Watershed Information System (GWIS) DEM Modeling and Terrains with ArcGIS (and other helpful items). Al Karlin, Ph.D. , GISP Mapping and GIS – SWFWMD 352-796-7211 x 4204. What is a Digital Elevation Model?. Digital Elevation Models, con’t. Digital Elevation Models, con’t.

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Geographic Watershed Information System (GWIS) DEM Modeling and Terrains with ArcGIS

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  1. Geographic Watershed Information System (GWIS) DEM Modeling and Terrains with ArcGIS (and other helpful items) Al Karlin, Ph.D. , GISP Mapping and GIS – SWFWMD 352-796-7211 x 4204

  2. What is a Digital Elevation Model?

  3. Digital Elevation Models, con’t.

  4. Digital Elevation Models, con’t.

  5. What is a Digital Elevation Model/Digital SURFACE Models (DSM) ?

  6. Digital Elevation Models/Digital SURFACE Models (DSM)

  7. What is a Digital Elevation Model/Digital TERRAIN Model (DTM) ? So…

  8. Three terms (DEM,DSM,DTM) for the same thing? Digital Elevation Model (DEM) Digital Surface Model (DSM) Digital Terrain Model (DTM)

  9. Not so easy … A Digital Elevation Model (DEM) is any DIGITAL representation of ground surface topography or terrain. Digital Surface Model (DSM) is a first surface view of the earth containing both location and elevation information. Digital Terrain Model (DTM), aka "bare earth" as it is often referred, is created by digitally removing all of the cultural features inherent to a DSM by exposing the underlying terrain. Representation is another issue: Raster or Triangular Irregular Network (TIN) (Interpolated TIN with faces) (TIN) Raster DEM (or Interpolated TIN)

  10. Then… With release 9.2 of ArcGIS, ESRI released a NEW data structure called: TERRAIN Dataset Terrains are a new dataset for ArcGIS 9.2. They live inside feature datasets in personal, file or SDE geodatabases. The other feature classes in the feature dataset can participate in the terrain or actually be embedded in the terrain, which means that the source data could be moved off-line after the creation of the terrain dataset. The graphic below illustrates how multiple types of feature classes can participate to generate TIN pyramids.

  11. So… What is a Terrain Dataset? • A terrain dataset is a multiresolution, TIN-based surface built from measurements • stored as features in a geodatabase. They're typically made from LIDAR, SONAR, • and photogrammetric sources. Terrains reside in the geodatabase, inside feature • datasets with the features used to construct them. • Terrains have participating feature classes and rules, similar to topologies. Common • feature classes that act as data sources for terrains include: • Multipoint feature classes of 3D mass points created from a data source such as • LIDAR or SONAR • 3D point and line feature classes created on photogrammetric workstations using stereo • imagery • Study area boundaries used to define the bounds of the terrain dataset • The terrain dataset's rules control how features are used to define a surface. For • example, a feature class containing edge of pavement lines for roads could participate • with the rule that its features be used as hard breaklines. This will have the desired • effect of creating linear discontinuities in the surface.

  12. Terrains con’t Rules also indicate how a feature class participates through a range of scales. The edge of pavement features might only be needed for medium to large-scale surface representations. Rules could be used to exclude them from use at small scales, which would improve performance. A terrain dataset in the geodatabase references the original feature classes. It doesn't actually store a surface as a raster or TIN. Rather, it organizes the data for fast retrieval and derives a TIN surface on the fly. This organization involves the creation of 'terrain pyramids' that are used to quickly retrieve only the data necessary to construct a surface of the required level of detail (LOD) for a given area of interest (AOI) from the database. The appropriate pyramid level is used relative to the current display scale.

  13. Some ESRI Terrain Gottcha’s 1- NGVD to NAVD Conversion – LiDAR data are referenced to NAVD88 but ERP data are referenced to NGVD29 2- Size limits for Terrains: 2 GB (20 million points) in pGDB 1 TB (several hundred million points) in fGDB unlimited in ArcSDE 3- Limited to file-based GeoDatabases – Large Terrains will only work in a file-based GDB: ArcINFO/ArcEditor only 4- Size limits for TIN – 15 – 20 million nodes (32 bit processing) 5- Size limits for Rasters/Grids – 4,000,000 x 4,000,000 cells (at 5’x5’ cells, that amounts to a watershed no larger than 4000 x 4000 miles (quite large, but…) 6- ArcHydro processing limits – recommended for DEMs up to 20,000 x 20,000 (at 5’x5’ cells = 400 sq. miles) 7- Raster can be stored in a fGDB – but must be converted to a Grid (external to the fGDB) for processing!

  14. General LiDAR/Terrain Workflow Step 1 – Create a file-based Geodatabase Step 2 – Import/Create Feature Classes Step 3 – Convert LAS to Multipoints Step 4 – Build the Terrain Step 5 – Extract a Digital Elevation Model from the Terrain

  15. Step 4 - Creating a Terrain Dataset In ArcToolbox In ArcCatalog Or… Notice the GWIS schema

  16. In ArcToolbox Step 1 – Create the Terrain in a GDB SWFWMD recommends = 4

  17. In ArcToolbox Step 2 – Add Feature Classes to the Terrain Make sure to populate the: height field SF_Type field

  18. In ArcToolbox Step 3 – Add Pyramids to the Terrain Level 1: 0.25 6800 Level 2: 1.00 12000

  19. In ArcToolbox Step 4 – Build the Terrain

  20. In ArcCatalog Use the Terrain Wizard by right-clicking on the GDB

  21. In ArcCatalog Select Feature Classes Adjust Breaklines SF_Type Post Spacing = 4 Set Pyramid Levels Confirm Settings and Finish

  22. Terrain of the Little Withlacoochee Watershed

  23. Step 5 - Extracting a Digital Elevation Model General Work Flow: Step 1 – Set Environment Variables Step 2 – Run the Terrain to Raster GP Tool Step 3 – Wait Step 4 – Check the DEM

  24. In ArcToolbox (either in ArcMap or ArcCatalog) Right-click on the blank space (anywhere) in the Toolbox

  25. In ArcToolbox Highlight the “Environments…” option to open the dialog box

  26. In ArcToolbox Use the Folder next to the “Extent” option to navigate to the LIDARTILES Feature Class in the fGDB, and note the extent values. These need to be multiples of 5’, so adjust to…

  27. In ArcToolbox Make them exact multiples of 5’ (ArcGIS likes to cheat; do not let it.) Also, check on Current workspace and other variables as needed.

  28. In ArcToolbox Make sure path is DOS 8.3 compliant Make sure this is “0” The Terrain to Raster GP Tool will open the dialog box. Fill in all fields as indicated above. At this time, it is also a good idea to check the Environments… to make certain that they are set to the proper coordinates. Then “OK” (and wait again!)

  29. DEM of the Little Withlacoochee Watershed

  30. Perfect alignment of adjacent DEMs Aripeka DEM Indian Creek DEM

  31. Some more ESRI Terrain/DEM Gottcha’s 1- File-based GDB can be read, but not processed in Arcview, so an ArcINFO or ArcEditor license is required. 2- An ESRI 3D Analyst license is needed to build Terrains and extract DEMs. 3- Based on a Pentium 4HT processor (3GHz with 4GB RAM), it takes approximately 0.75 - 1 hr/100 pyramids to construct a Terrain. The progress bar indicates the number of pyramids in the Terrain, so judge accordingly. 4- Based on a Pentium 4HT processor (3GHz with 4 GB RAM), it takes approximately 0.75 - 1 hr/300 pyramids to construct a DEM. 5- SWFWMD experience shows that the Terrain to Raster interpolator appears VERY sensitive to breaklines with “null” elevations. This will cause the interpolator (Linear or Natural Neighbors) to stop and produce an incomplete DEM. 6- The GP tools in ArcToolbox can be used to delete feature classes from the Terrain, but sometimes it may be easier to remake the Terrain. 7- The Terrain to Raster interpolator is VERY sensitive to path and file names. Paths can not have spaces, underscores, etc. and file names can have no more than 11 DOS-compliant characters.

  32. And even more ESRI Terrain/DEM Gottcha’s 8- ESRI Rasters CAN exist within a GDB, BUT during processing they are converted to Grids. So, although you CAN it is not a good idea to keep DEMs in the GDB; keep them in their own, separate location. 9- It is a good idea to check the DEM after it is generated to make certain that the cell size is correct and that adjacent DEMs align properly as this will help eliminate slivers along the catchment boundaries. 10- Make sure that you have sufficient FREE DISK space BEFORE you begin processing any Terrains or DEMs. Our “rule of thumb” is to have 2GB free for each 1GB in the GDB. 11- Unless you have a VERY fast, dual core or quad core computer workstation, building Terrains or DEMs is a dedicated task. Try to let these run overnight or some other low-use time. 12- Make sure that the connection to the ESRI licenser server is not broken!

  33. Questions…, comments…, thoughts to share… Al.Karlin@SWFWMD.State.Fl.US – x4204 Ekaterina.Fitos@SWFWMD.State.Fl.US - x 4219 Cheryl.Glenn@SWFWMD.State.Fl.US – x 4224

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