floodplain modeling with lidar-derived terrain

Floodplain Modeling and Floodplain Modeling and Delineation with LiDAR-Derived Delineation with LiDAR-Derived

TerrainTerrain

Wisconsin Dept. of Natural ResourcesWisconsin Dept. of Natural Resources

Bureau of Watershed ManagementBureau of Watershed Management

FEMA Risk MAP ProgramFEMA Risk MAP Program

Katie McMahan – GIS Data ManagerKatie McMahan – GIS Data Managerkatie.mcmahan@wi.govkatie.mcmahan@wi.gov

Chris Olds – Floodplain EngineerChris Olds – Floodplain Engineerchristopher.olds@wi.govchristopher.olds@wi.gov

Presentation OverviewPresentation Overview

LiDAR LiDAR

– ProductsProducts

TerrainTerrain

– Required feature classes (surface feature types)Required feature classes (surface feature types)

– Process of buildingProcess of building

– Possible issues that may arisePossible issues that may arise

Floodplain delineationFloodplain delineation

– HEC-GeoRAS geometry to create input feature classesHEC-GeoRAS geometry to create input feature classes

– HEC-RAS to determine flood elevationsHEC-RAS to determine flood elevations

– HEC-GeoRAS mapping to delineate raw floodplain polygonsHEC-GeoRAS mapping to delineate raw floodplain polygons

– Smoothing and merging multiple floodplainsSmoothing and merging multiple floodplains

Products received Products received vs. vs.

what we usewhat we use Bare earthBare earth

• DEMDEM• DWGDWG• LASLAS• SHPSHP

BreaklinesBreaklines• DWGDWG• SHPSHP

ContoursContours• DWGDWG• SHPSHP

Point cloudPoint cloud• DWGDWG• SHPSHP• LASLAS

MetadataMetadata• Important elements:Important elements:

– ProjectionProjection– Average point Average point

spacingspacing

Terrain OverviewTerrain Overview

TIN-based dataset that is a representation of TIN-based dataset that is a representation of

input feature classesinput feature classes

Terrain is editable—can remove, add and Terrain is editable—can remove, add and

replace the data referencedreplace the data referenced

Steps to building terrainSteps to building terrain

Create a File Geodatabase

The personal geodatabase has a 2GB size limit, too small to handle an entire County’s worth of LIDAR data.

Feature DatasetFeature Dataset In ArcCatalog, create a new In ArcCatalog, create a new

‘Feature Dataset’. This will ‘Feature Dataset’. This will

house all data used to and house all data used to and

create your terrain. create your terrain.

Choose the coordinate system Choose the coordinate system

the LiDAR data is in; most the LiDAR data is in; most

often a County coordinate often a County coordinate

system.system.

Make sure the linear unit of Make sure the linear unit of

measurement (ft or meter) is measurement (ft or meter) is

defined correctly for both the defined correctly for both the

projected and vertical projected and vertical

coordinate systemscoordinate systems

Feature DatasetFeature Dataset

Import all of the features that will be used to create the Import all of the features that will be used to create the

terrain into feature dataset. terrain into feature dataset.

– LIDAR multipoints (created from LAS files)LIDAR multipoints (created from LAS files)

– BreaklinesBreaklines

– Hard-clip polygons (usually a buffered polygon of the Hard-clip polygons (usually a buffered polygon of the

County boundary) County boundary)

Determine Average Point Spacing of bare earth pointsDetermine Average Point Spacing of bare earth points

Average Point SpacingAverage Point Spacing

From LiDAR metadata or using the 3D Analyst Point From LiDAR metadata or using the 3D Analyst Point File Information toolFile Information tool

Average Point SpacingAverage Point Spacing

Bare Earth MultipointsBare Earth Multipoints

Multipoints should be Multipoints should be

spatially clusteredspatially clustered

Processing terrain Processing terrain

countywide: break countywide: break

down by township or down by township or

rangerange

Recent example: Recent example:

– 830 LAS files830 LAS files

– LiDAR points: LiDAR points:

2,074,016,0002,074,016,000

LAS to MultipointLAS to Multipoint

LAS to MultipointLAS to Multipoint

BreaklinesBreaklines Lines with height recorded at each vertex Lines with height recorded at each vertex

Represents some sort of interruption in the landscape:Represents some sort of interruption in the landscape:

– Natural landforms (ridges, valleys) - SNatural landforms (ridges, valleys) - S

– Edge of pavement - HEdge of pavement - H

– Roadways - HRoadways - H

– **Water lines (streams, rivers, canals, shorelines) - H**Water lines (streams, rivers, canals, shorelines) - H

Eliminates interpolation of LiDAR points across the breakline; becomes the edge of the Eliminates interpolation of LiDAR points across the breakline; becomes the edge of the

trianglestriangles

Hard lineHard line: represents distinct break in the slope: represents distinct break in the slope

Soft lineSoft line: allows the adding of edges without affecting the shape of the surface: allows the adding of edges without affecting the shape of the surface

**Taken from ArcGIS **Taken from ArcGIS Desktop Help 9.3.1Desktop Help 9.3.1

Hard Hard BreaklinesBreaklines

Clip PolygonClip Polygon Define boundaries for interpolating a terrain surface with an Define boundaries for interpolating a terrain surface with an

irregular shape—data that falls outside the polygon is excludedirregular shape—data that falls outside the polygon is excluded

Study area boundary Study area boundary

No height sourceNo height source

**Taken from ArcGIS **Taken from ArcGIS Desktop Help 9.3.1Desktop Help 9.3.1

Build Build TerrainTerrain

Choose FeaturesChoose Features

Pyramid TypePyramid Type

Terrain PyramidsTerrain Pyramids Levels of detail to improve efficiency—a form of scale dependent generalization Levels of detail to improve efficiency—a form of scale dependent generalization

(thinning points)(thinning points)

Z-ToleranceZ-Tolerance: uses a vertical tolerance in the definition of the terrain resolution at : uses a vertical tolerance in the definition of the terrain resolution at

the specified scale (relative to vertical accuracy at full-resolution)the specified scale (relative to vertical accuracy at full-resolution)

– Used to improve analysis performance through a range of scalesUsed to improve analysis performance through a range of scales

Window SizeWindow Size: resolution defined by window : resolution defined by window

– Used for improving interactive display performance (specific vertical Used for improving interactive display performance (specific vertical

accuracies is not a primary concern)accuracies is not a primary concern)

When exporting rasters or TINs, the entire feature dataset is used (no difference When exporting rasters or TINs, the entire feature dataset is used (no difference

from actual surface).from actual surface).

**Not necessary for terrain use at constant and large display scale, i.e. raster **Not necessary for terrain use at constant and large display scale, i.e. raster

production for large-scale applications or for data storage purposes; time production for large-scale applications or for data storage purposes; time

intensive, no reason to incur the processing cost.intensive, no reason to incur the processing cost.

Z-Tolerance Z-Tolerance

Must generate Must generate one pyramid no one pyramid no matter which matter which type you choosetype you choose

Summary of terrain settingsSummary of terrain settings

Terrain OverviewTerrain Overview

Coarsest representation of the terrain datasetCoarsest representation of the terrain dataset

For fast drawing at small scalesFor fast drawing at small scales

Set only those feature classes that must be Set only those feature classes that must be

represented in the overview to ‘Yes’represented in the overview to ‘Yes’

Terrain OverviewTerrain Overview

**Taken from ArcGIS Desktop Help 9.3.1**Taken from ArcGIS Desktop Help 9.3.1

3D Analyst Conversion Tool: 3D Analyst Conversion Tool: Terrain to RasterTerrain to Raster

Float will allow the value of the pixel to be in decimal format

5-foot cell size

Conversion to rasterConversion to raster

Countywide 5-foot gridCountywide 5-foot grid

3D Analyst license is required; 3D Analyst license is required;

Choose LAS over ASCII (if available); Choose LAS over ASCII (if available);

The data involved should be contiguous;The data involved should be contiguous;

Data should be in a projected coordinate system;Data should be in a projected coordinate system;

More straightforward to have the z-values in same unit of measure as the x,y More straightforward to have the z-values in same unit of measure as the x,y

values; values;

Use data gathered using the same data collection specifications and accuracy Use data gathered using the same data collection specifications and accuracy

requirements. requirements.

Use the fewest feature classes possible; no large single point feature classes. Use the fewest feature classes possible; no large single point feature classes.

Use a multipoint feature class for large point collections (e.g., anything over Use a multipoint feature class for large point collections (e.g., anything over

500,000).500,000). Allow plenty of time for processing large areas of terrainAllow plenty of time for processing large areas of terrain

Tips for building terrainTips for building terrain

ComplicationsComplications

Missing tiles = visual confirmation of the terrain Missing tiles = visual confirmation of the terrain

– requires cooperation with original vendor to gather requires cooperation with original vendor to gather

complete datasetcomplete dataset

Incorrect height values = visual confirmation of the terrain (i.e. Incorrect height values = visual confirmation of the terrain (i.e.

giant spikes or dips in elevation) or in floodplain delineation giant spikes or dips in elevation) or in floodplain delineation

‘gaps’‘gaps’

– may result in the need for additional ground truthing or may result in the need for additional ground truthing or

survey cross sections along the streamsurvey cross sections along the stream

Floodplain delineation ‘gaps’Floodplain delineation ‘gaps’

Floodplain DelineationFloodplain Delineation

HEC-GeoRASHEC-GeoRAS

Creating Feature ClassesCreating Feature Classes

Creating Feature ClassesCreating Feature Classes

Populating Feature ClassesPopulating Feature Classes

Layer SetupLayer Setup

Layer SetupLayer Setup

Extracting Topological DataExtracting Topological Data

HEC-RASHEC-RAS

Importing GIS data into HEC-RASImporting GIS data into HEC-RAS

Cross section in HEC-RASCross section in HEC-RAS

Cross Section 3D ViewCross Section 3D View

Cross section results in HEC-RASCross section results in HEC-RAS

Flood profile in HEC-RASFlood profile in HEC-RAS

Exporting HEC-RAS results Exporting HEC-RAS results into GIS filesinto GIS files

Floodplain delineation using Floodplain delineation using HEC-GeoRASHEC-GeoRAS

Floodplain Delineation Floodplain Delineation Layer SetupLayer Setup

Importing HEC-RAS dataImporting HEC-RAS data

Importing HEC-RAS dataImporting HEC-RAS data

HEC-RAS data back in GISHEC-RAS data back in GIS

Flood Inundation MappingFlood Inundation Mapping

Water surface TIN generationWater surface TIN generation

Flood Inundation MappingFlood Inundation Mapping

Flood Inundation MappingFlood Inundation Mapping

Flood depth gridFlood depth grid

Floodplain boundary polygonFloodplain boundary polygon

Floodplain boundary polygon smoothingFloodplain boundary polygon smoothing

Floodplain boundary polygon smoothingFloodplain boundary polygon smoothing

Merging floodplain feature classesMerging floodplain feature classes

QUESTIONS?QUESTIONS?