Geographic Watershed Information System/ArcHydro – 1 February 2008 Geographic Watershed...

Geographic Watershed Information System/ArcHydro – 1 February 2008

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

(and other helpful items)

Al Karlin, Ph.D. , GISPMapping and GIS – SWFWMD

352-796-7211 x 4204


What is a Digital Elevation Model?


Digital Elevation Models, con’t.


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


Digital Elevation Models/Digital SURFACE Models (DSM)


What is a Digital Elevation Model/Digital TERRAIN Model (DTM) ?

So…


Digital Terrain Model (DTM)

Digital Elevation Model (DEM)

Digital Surface Model (DSM)

Three terms (DEM,DSM,DTM) for the same thing?


Not so easy …

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.

A Digital Elevation Model (DEM) is any DIGITAL representation of ground surface topography or terrain.

Representation is another issue: Raster or Triangular Irregular Network (TIN)

(TIN)(Interpolated TIN

with faces)Raster DEM (or Interpolated TIN)


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.


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.

So… What is a Terrain Dataset?


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.

Terrains con’t


1- NGVD to NAVD Conversion – LiDAR data are referenced to NAVD88but 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 fGDBunlimited in ArcSDE

3- Limited to file-based GeoDatabases – Large Terrains will only workin 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 than4000 x 4000 miles (quite large, but…)

6- ArcHydro processing limits – recommended for DEMs up to 20,000 x20,000 (at 5’x5’ cells = 400 sq. miles)

7- Raster can be stored in a fGDB – but must be converted to a Grid (externalto the fGDB) for processing!

Some ESRI Terrain Gottcha’s


General LiDAR/Terrain Workflow

Grid DatasetFeature Class

DatasetFunction

Legend

Optional FunctionObject Class

(Table)

LAS (LiDAR data)Terrain Break lines including:

HYDROGRAPHICFEATURESROADS

SOFTFEATURESISLANDS

WATERBODIESCOASTALSHORELINES

OBSCUREDVEG

NHD FlowlinesNHD Water Bodies and Swamps

PROJECT AREA (Polygon)

STEP 1 – CREATING FILE GEODATABASECreate File GeoDatabase and a Feature DataSet

When defining the FDS, IMPORT the Spatial Data Reference from the Terrain Break lines. Be certain to assign the correct Vertical Datum and check to insure that the units are correct

STARTING TERRAIN FGDBFROM PROVIDER

STEP 2 – IMPORTING FEATURE CLASSESImport the Feature Classes from the Starting Data Sets into the

GeoDatabase. Use IMPORT as either single or multiple features, making certain that all data are in the same projection

system as that defined for the fGDB.

STEP 3 – CONVERTING LAS (LIDAR) DATAEngage the 3D Analyst Extension and from the 3D Analyst ToolBox, choose Conversion|From File|LAS to Multipoint

Choose the LAS Files Average Spacing = 6 (or other appropriate value) Input Class Codes = 2,10,11IMPORT the Spatial Reference from the fGDB checking for the

correct Vertical Datum and units

In ArcCatalog

In ArcCatalog

In ArcCatalog

Terrain (Multipoint) Feature Class(MASSPOINTS)

STEP 4 – BUILD THE TERRAIN FEATURE CLASSCreate a new Terrain Feature Class with the following:

Terrain Name = XXTerrain (where XX is anything)Select the Feature Classes (at minimum)

Feature Class = SFtype MASSPOINTS = Mass Points HYDROGRAPHICFEATURES = Hard Line ROADS = Hard Line ISLANDS = Hard Fill Value SOFTFEATURES = Soft Line WATERBODIES = Hard Replace COASTALSHORELINE = Hard Line [PROJECT AREA = Soft Clip] if not set in Environment

Calculate Pyramid Properties as:0.25 68001.00 12000

Check on the Advanced Bounds Settings (Button) and make certain that the max value is set for the minimum pyramid level.

Set Environment Variables as needed to insure appropriate1- workspace and scratch space

2- Extents (as necessart)3- Units and Coordinate system

TERRAIN FEATURE CLASS(XXTerrain)

In ArcCatalog

STEP 5 – CHECK THE TERRAIN FCOpen ArcMap and Add the TerrainRight-Click on Terrain|Properties

Select Symbology – Set the Classification Method for Elevation to “Natural Breaks” and change the number of classes to 20.

Select the desired Color RampVisually Check Terrain for consistency

In ArcMap (OPTIONAL)

STEP 6 – CREATE A FLOATING POINT OR INTEGER RASTER FOR ARCHYDRO PROCESSING

Close ArcMapFrom the 3D Analyst Toolbox, choose:

Conversion|From Terrain|Terrain to Raster Use the XXTerrain, output a raster (outside of fGDB) using

Method = Natural_NeighborsCellSize = 5.0

Pyramid Level Resolution = 0Set Environment Variables as needed

In ArcCatalog

FLOATING POINT GRID FOR

ARCHYDRO

LAS (LiDAR) Data Terrain Processing Workflow for ArcHydro

NOTE: This step will require approximately 5-20 minutes per square mile of terrain. The unit time is dependent on the number of break lines and other factors.

NOTE: The import time is approximately 30 seconds per 1 sq. mile tile.

NOTE: Import time for the Multiclass features is about 2 – 3 minutes for a 15 sq. mile. watershed.

NOTE: This is the longest step. It can take a few minutes to several hours depending on the Cell Size specified. The smaller the cell size, the greater the processing time.

OTHER FEATURE CLASSES TO

INCORPORATE

LAS Data files

INTEGER GRID FOR ARCHYDRO

(OPTIONAL OUTPUT)

Choice made during Conversion

Step 1 – Create a file-based Geodatabase

Step 2 – Import/Create Feature Classes

Step 3 – Convert LAS to Multipoints

Step 5 – Extract a Digital Elevation Model from the Terrain

Step 4 – Build the Terrain


Step 4 - Creating a Terrain Dataset

In ArcToolbox In ArcCatalog

Or…

Notice the GWIS schema


In ArcToolbox

Step 1 – Create the Terrain in a GDB

SWFWMD recommends = 4


In ArcToolbox

Step 2 – Add Feature Classes to the Terrain

Make sure to populate the:height field

SF_Type field


In ArcToolbox

Step 3 – Add Pyramids to the Terrain

Level 1: 0.25 6800Level 2: 1.00 12000


In ArcToolbox

Step 4 – Build the Terrain


In ArcCatalog

Use the Terrain Wizard by right-clicking on the GDB


In ArcCatalog

Post Spacing = 4

Select Feature Classes Adjust Breaklines SF_Type

Set Pyramid Levels

Confirm Settings and Finish


Terrain of the Little Withlacoochee Watershed


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


In ArcToolbox (either in ArcMap or ArcCatalog)

Right-click on the blank space (anywhere) in the Toolbox


In ArcToolbox

Highlight the “Environments…” option to open the dialog box


In ArcToolbox

Use the Folder next to the “Extent” option to navigate to the LIDARTILESFeature Class in the fGDB, and note the extent values. These need to be

multiples of 5’, so adjust to…


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.


In ArcToolbox

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!)

Make sure path is DOS 8.3 compliant

Make sure this is “0”


DEM of the Little Withlacoochee Watershed


Perfect alignment of adjacent DEMs

Aripeka DEM

Indian CreekDEM


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 inthe Terrain, so judge accordingly.

4- Based on a Pentium 4HT processor (3GHz with 4 GB RAM), it takesapproximately 0.75 - 1 hr/300 pyramids to construct a DEM.

5- SWFWMD experience shows that the Terrain to Raster interpolatorappears 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 classesfrom the Terrain, but sometimes it may be easier to remakethe Terrain.

7- The Terrain to Raster interpolator is VERY sensitive to path andfile names. Paths can not have spaces, underscores, etc. andfile names can have no more than 11 DOS-compliant characters.


And even more ESRI Terrain/DEM Gottcha’s

8- ESRI Rasters CAN exist within a GDB, BUT during processingthey are converted to Grids. So, although you CAN it isnot a good idea to keep DEMs in the GDB; keep them intheir 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 DEMsalign properly as this will help eliminate slivers along thecatchment boundaries.

10- Make sure that you have sufficient FREE DISK space BEFOREyou begin processing any Terrains or DEMs. Our “ruleof thumb” is to have 2GB free for each 1GB in the GDB.

11- Unless you have a VERY fast, dual core or quad corecomputer workstation, building Terrains or DEMs is adedicated task. Try to let these run overnight or someother low-use time.

12- Make sure that the connection to the ESRI licenser server is not broken!


Questions…, comments…, thoughts to share…

[email protected] – [email protected] - x [email protected] – x 4224

Geographic Watershed Information System/ArcHydro – 1 February 2008 Geographic Watershed...

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