Denver DFIRM Depth Grid Exercise
The purpose of this exercise is to demonstrate the geoprocessing steps to create a depth grid (DG)
utilizing Digital Flood Insurance Rate Map (DFRIM) and ESRI ArcMap spatial analyst toolset.
A few data considerations to keep in mind while working through the geoprocessing steps to creating a
depth grid:
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Is all of your data conforming to the same projection? If requisite datasets are projected
differently, you will encounter errors in your geoprocessing procedure. It is recommended that
all datasets be projected into Geographic Coordinate System North American Datum 1983
(GCS_NAD83).
What vertical units is your terrain data displayed? Feet? Meters? If your terrain data is in
meters you will need to use the Raster Calculator to convert your terrain dataset to feet.
What vertical datum are the data? NAVD88? NGVD29? The vertical datum of terrain data and
DFIRM data need to match. Most data will be delivered in NAVD88. In the rare case data are
collected and delivered in NGVD29, it will need to convert the data using VERTCON
(http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl). Detailed steps in regards to
converting vertical datum using VERTCON are explained in Appendix A of the Coastal Flood Loss
Atlas SOP, which can be found here:
https://content.femadata.com/MOTF/SOPs/CFLA%20SOP%20Draft%202012.pdf
Required Geographic Data
Digital Flood Insurance Rate Map (DFIRM)
DFIRMs are the digital version of a Flood Insurance Rate Map (FIRM), which show a community’s base
flood elevations, flood zones, and floodplain boundaries for the 1% and 0.2% annual chance flood
hazard zones. All data used to produce the FIRM are stored in the DFIRM Database. A portion of the
DFIRM Database is packaged as the NFHL; available from the FEMA MSC.
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For this exercise, we will be using several features found in the DFIRM Database for Denver.
Navigate to the FEMA Maps Service Center (http://msc.fema.gov/portal) and download the
current effective data for the City and County of Denver (NFHL_080046; Last Study Effective
Date: 11/20/2013; Last LOMR Effective Date: 06/19/2015).
Terrain Data (DEM)
Terrain data can be acquired from many sources. The most common venue for obtaining elevation
datasets are through the USGS National Map Viewer (http://naionalmap.gov/viewer.html). The
National Elevation Dataset (NED) is available nationally at resolutions of 1 arc-second (about 30 meters),
1/3 arc second (about 10 meters), and in 1/9 arc-second (about 3 meters).
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In addition, high resolution LiDAR (Light Detection and Ranging) data terrain data (1-3 meter resolution)
may be available through local municipalities within your project area. A good place to start looking for
information on local LiDAR collections is the US Interagency Elevation Inventory
(http://coast.noaa.gov/inventory/#).
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For this exercise, we will be using 1 meter LiDAR collected after the 2013 Colorado floods. Time
and level of analysis detail are prime factors in determining the resolution of terrain data to be
used in your analysis. For demonstration and time purposes only a 3 square mile subset. DEM –
FEMA MOTF FTP Site
(https://content.femadata.com/MOTF/MOTF_Workshop/DFIRM_Based_DG_Exercise/DEM_ft.zi
p)
Area of Interest (AOI)
To ensure everyone is able to complete the exercise in a reasonable time frame, an AOI has been
provided. Area of Interest for Exercise – FEMA MOTF FTP Site
(https://content.femadata.com/MOTF/MOTF_Workshop/DFIRM_Based_DG_Exercise/Denver_DG_AOI.z
ip)
Fundamental Depth Grid Logic
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Depth Grid Exercise
These steps assume that the analyst has background knowledge of the ArcMap User Interface,
understands geographic data workflow practices, and understanding of the interaction of different
geographic data types. It is suggested that a temporary/scratch workspace be created to organize all
intermediate datasets.
Technical details for DG creation, including data management workflows, can be found in the MOTF
Level 2 Site Specific Flood Model SOP downloadable at the following location:
https://content.femadata.com/MOTF/SOPs/Hazus_Level_2_Site_Specific_Flood_Analysis.pdf.
For this exercise, create a depth grid based off of the City and County of Denver DFIRM for an area along
the South Platte River near Evans Ave and I-25. To do this, the DFIRM Database will need to be
downloaded from the FEMA Map Service Center, and the DEM and the AOI can be found the FEMA
MOTF FTP site. The DEM has been clipped to the area of interest.
Step 1. Download data for the City and County of Denver, Colorado
a. DFIRM – FEMA Maps Service Center (http://msc.fema.gov/portal) for the City and County of
Denver (NFHL_080046; Last Study Effective Date: 11/20/2013; Last LOMR Effective Date:
06/19/2015).
b. DEM – FEMA MOTF FTP Site
(https://content.femadata.com/MOTF/MOTF_Workshop/DFIRM_Based_DG_Exercise/DEM_
ft.zip)
c. Area of Interest for Exercise – FEMA MOTF FTP Site
(https://content.femadata.com/MOTF/MOTF_Workshop/DFIRM_Based_DG_Exercise/Denv
er_DG_AOI.zip)
Step 2. Preparing the data
a. Ensure all data are projected in the Geographic Coordinate System North American Datum
b. Clip the S_FLD_HAZ_AR to the AOI
c. Select by Attribute and export to a unique shapefile.
i. AE Flood Zones
ii. AO Flood Zones
iii. AH Flood Zones
d. Select by Attribute SFHA=T, export to create a new shapefile
i. Use the Dissolve geoprocessing tool to create the inundation boundary
e. Clip the Xs (cross sections) by the AE Flood Zone
f. Use the Feature Vertices to Point tool to convert the cross section line segments into points.
i. Set the Point Type to All
Step 3. Interpolation of the Water Surface
a. Using the Inver Distance Weighted (IDW) tool, interpolate a water surface from the point
shapefile created during Step 2-F
i. Output cell size needs to match the cell size of the DEM
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ii. Set the Processing Extent to the Denver_DG_AOI.shp
iii. Set the Snap to Rater environment to the DEM
Step 4. Creating the AE Zone Depth Grid
a. Subtract the Water Surface from the DEM
i. Utilizing the interpolated water surface from the previous step, subtract this surface
from the DEM (DG_ft) using the Raster Calculator or Minus spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
 If any geoprocessing errors arise at this point, take a step back and read the
data consideration mentioned previously. Double check that your data
conforms to some of these considerations.
 At this step, take time to critically think about why the water surface would
need to be subtracted from the ground surface.
b. Remove Negative Values
i. Utilizing the raster output from the previous step, remove the negative values from
the dataset using the Raster Calculator or Greater Than Equal spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
c. Convert Raster to Polygon
i. The raster output from the previous step is a “True/False” raster. Where “1”
represents a true statement and “0” represents a false statement.
ii. To derive a flood extent, the “1” or “true” values need to be extracted. Use the
Raster to Polygon conversion tool to convert this raster into a vector dataset for
further processing.
 Be sure to uncheck the “Simplify Polygons” box.
d. Extract Flood Boundary
i. Using the output vector polygon form the previous step. Export the “1” or “true”
values from this polygon by using either Select by Attributes or a Definition Query.
ii. Dissolve the flood polygon.
iii. Clip the dissolve boundary by the AE flood zone shapefile created Step 2-C
e. Extract Depth Grid
i. Utilizing the flood boundary polygon created in the previous step, create a flood
depth grid using the Extract by Mask spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
f. 0.1 ft depth infill
i. Use the output vector polygon from Step 4-D.iii and the erase tool to create a
feature of the areas not inundated in the previous steps.
g. Extract 0.1 ft Depth Grid
i. Utilizing the flood boundary polygon created in the previous step, create a flood
depth grid using the Extract by Mask spatial analyst tool.
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 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
h. Finalizing the AE Flood Zone Depth Grid
ii. Use the Mosaic to New Raster tool to create the final AE Flood Zone Depth Grid.
 Using the two depth grids created in parts E and G
Step 5. Creating the AH Zone Depth Grid
a. The AH Flood Zone has a constant water surface elevation (static BFE), and there have been
revision to the DFRIM Database, and errors can occur. Specifically in this case, there is no
static BFE associated with the S_FLD_HAZ_AR shapefile. If this happens, please refer to the
FIRM (use the FIRM Panel shapefile for reference) to determine the Static BFE, which is
5,256 feet.
i. With AH Zones, the water surface elevation predetermined at a static level,
therefore, it is not necessary to interpolate a water surface.
b. Create Water Surface
i. Utilizing the AH Flood Zone boundary created in the previous Step 2-C; Extract by
Mask from the DEM to being to creation of the water surface.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
ii. To finish the creation of the water surface, open Raster Calculator and enter the
following expression: ((RasterFromStep5-B.i * 0)+StaticBFEinFeet)
 In this case the static BFE is 5,256 feet, and this can be found in the attribute
table in the S_FLD_HAZ_AR feature
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
c. Subtract the Water Surface from the DEM
i. Utilizing the interpolated water surface from the previous step, subtract this surface
from the DEM (DG_ft) using the Raster Calculator or Minus spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
 If any geoprocessing errors arise at this point, take a step back and read the
data consideration mentioned previously. Double check that your data
conforms to some of these considerations.
d. Remove Negative Values
i. Utilizing the raster output from the previous step, remove the negative values from
the dataset using the Raster Calculator or Greater Than Equal spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
e. Convert Raster to Polygon
i. The raster output from the previous step is a “True/False” raster. Where “1”
represents a true statement and “0” represents a false statement.
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ii. To derive a flood extent, the “1” or “true” values need to be extracted. Use the
Raster to Polygon conversion tool to convert this raster into a vector dataset for
further processing.
 Be sure to uncheck the “Simplify Polygons” box.
f. Extract Flood Boundary
i. Using the output vector polygon form the previous step. Export the “1” or “true”
values from this polygon by using either Select by Attributes or a Definition Query.
ii. Dissolve the flood polygon.
iii. Clip the dissolve boundary by the AH flood zone shapefile created Step 2-C
g. Extract Depth Grid
i. Utilizing the flood boundary polygon created in the previous step, create a flood
depth grid using the Extract by Mask spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
h. 0.1 ft depth infill
i. Use the output vector polygon from Step 5-F.iii and the erase tool to create a
feature of the areas not inundated in the previous steps.
i. Extract 0.1 ft Depth Grid
i. Utilizing the flood boundary polygon created in the previous step, create a flood
depth grid using the Extract by Mask spatial analyst tool.
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
j. Finalizing the AH Flood Zone Depth Grid
iii. Use the Mosaic to New Raster tool to create the final AH Flood Zone Depth Grid.
 Using the two depth grids created in parts E and G
k. In this exercise, there is only one AO Flood Zone. In many cases, there are multiple AO
zones with different depths. A depth grid should be created for each separately.
Step 6. Creating the AO Zone Depth Grid
a. The AO Flood Zone has a constant water depth, generally below 3 feet. Each AO can be
unique, but the process is much simpler to create a depth grid for this zone.
i. With AO Zones, the water surface elevation is not necessary; therefore, it is not
necessary to interpolate a water surface.
ii. The water depth in the AO zone can be found in the S_FLD_HAZ_AR attribute table
b. Create Depth Grid
i. Utilizing the AH Flood Zone boundary created in the previous Step 2-C; Extract by
Mask from the DEM to being to creation of the water surface
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
c. Create Constant Depth Grid
i. To finish the creation of the depth grid, open Raster Calculator and enter the
following expression: ((RasterFromStep5-B.i * 0) + ConstantWaterDepth)
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 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
d. In this exercise, there is only one AO Flood Zone. In many cases, there are multiple AO
zones with different depths. A depth grid should be created for each separately.
Step 7. Creating the Final Depth Grid
a. Use the Mosaic to New Raster tool to create the final DFIRM Depth Grid.
i. Add the raster surfaces created in Step 4-H, Step 5-J, and Step 6-D
 Output cell size needs to match the cell size of the DEM
 Set the Snap to Rater environment to the DEM
Understanding the DFIRM Database
080046_20150619.zip – (080046 – Community ID | 20150619 – Date updated) this zip file contains all of
the necessary data to create and review a water surface.
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The National Flood Hazard Layer (NFHL) data incorporates all Flood Insurance Rate Map
(FIRM) databases published by the Federal Emergency Management Agency (FEMA),
and any Letters of Map Revision (LOMRs) that have been issued against those databases
since their publication date. It is updated on a monthly basis. The FIRM Database is the
digital, geospatial version of the flood hazard information shown on the published paper
FIRMs.
The FIRM Database depicts flood risk information and supporting data used to develop
the risk data. The primary risk classifications used are the 1-percent-annual-chance flood
event, the 0.2-percent-annual-chance flood event, and areas of minimal flood risk. The
FIRM Database is derived from Flood Insurance Studies (FISs), previously published
FIRMs, flood hazard analyses performed in support of the FISs and FIRMs, and new
mapping data, where available. The FISs and FIRMs are published by FEMA.
The NFHL is available as State or US Territory data sets. Each State or Territory data set
consists of all FIRM Databases and corresponding LOMRs available on the publication
date of the data set.
The specification for the horizontal control of FIRM Databases is consistent with those
required for mapping at a scale of 1:12,000. This file is georeferenced to the Earth's
surface using the Geographic Coordinate System (GCS) and North American Datum of
1983.
Data included (bold features are the most often used for depth grid creation)
 080046_20150619_metadata.xml – Metadata
 L_COMM_INFO.dbf – information about each community on the FIRM.
 L_SOURCE_CIT.dbf – Listing and description of the sources of information used
in the FIRM or referenced in the FIS report bibliography.
 L_XS_ELEV.dbf – Information about the hydrologic model at each mapped cross
section and those cross sections referenced in the FIS report Floodway Data
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Tables. This table is used to generate the Floodway Data Tables and contains
lettered cross sections.
S_BFE – Shapefile with the location and attributes for Base Flood Elevations
(BFEs) shown on FIRM.
S_FIRM_PAN – Shapefile with location and attributes for FIRM hardcopy map
panels.
S_FLD_HAZ_AR – Shapefile with the location and attributes for flood insurance
risk zones on the FIRM.
S_FLD_HAZ_LN – Shapefile with the location and attributes for the boundaries
of flood insurance risk zone on the FIRM.
S_GEN_STRUCT – Shapefile with the location and attributes for flood control
structures shown on the flood profile and FIRM.
S_LEVEE – Shapefile with the location of levee centerlines, floodwalls, and
closure structures protecting accredited and provisionally accredited levees.
S_LOMR – Shapefile with the location and attributes for Letter of Map Revisions
(LOMRs) not yet incorporated on the FIRM. Used only as part of the national
Flood Hazard Layer (NFHL).
S_PLSS_AR – Shapefile with the location and attributes of sections, townships,
and ranges on the FIRM
S_POL_AR – Shapefile with the location and attributes for political jurisdictions
shown on the FIRM.
S_PROFIL_BASLN – Shapefile with the location and attributes for profile baseline
and stream centerline features for the flood risk project area.
S_WTR_LN – Shapefile with the location and attributes for hydrography features
shown on FIRM as lines
S_XS – Shapefile with the location and attributes for cross section lines in the
area covered by the FIRM. This layer must contain all cross sections in a
model, not just the lettered cross sections.
STUDY_INFO.dfb – General information about the FRIM.
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