Method 4: DFIRM Shapefiles

Digital Flood Rate Insurance Maps are available to download from FEMA for $10. They’ve offered a few free samples and Fairfax City happens to be one of them.

The data comes in several formats including shapefile. ArcGIS Explorer can read shapefiles. However, it will not let you add them to your map unless they have a defined projection.

The shapefiles in the Fairfax City DFIRM that I downloaded didn’t have their projections defined. I would assume this is the case with all of them. Luckily, they tell you the projection in the metadata. And luckily, projections can be defined with a file you can create using any text editor.

To find the projection, open the _metadata file in the Document folder. If you scroll down about 2/3 of the way you’ll find the Spatial_Reference_Information section. The most important parts are the Grid_Coordinate_System_Name, UTM_Zone_Number, and Horizontal_Datum_Name. The Fairfax City DFIRM is in Universal Transverse Mercator (UTM) Zone 18, NAD 1983 datum.

I used that information to have ArcGIS desktop create a projection definition file in the format used by all ESRI GIS software (including ArcGIS Explorer). It looks like this:
PROJCS["NAD_1983_UTM_Zone_18N",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",
SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-75.0],PARAMETER["Scale_Factor",0.9996],
PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
All you need to do is copy and paste that text into a text editor, remove any spaces, then save it as a .prj file. The name before the file extension should match the shapefile you are trying to use. The main DFIRM shapefile is S_Fld_Haz_Ar.shp, the flood hazard zone areas. So the projection definition file should be called S_Fld_Haz_Ar.prj

If you don’t want to copy and paste you can download it and put it the same folder with the shapefile.

FEMA uses UTM for all of its DFIRMs, but they do not always use the same datum. If you download a different one from Fairfax City, you will need to check the metadata for the UTM Zone Number and whether the datum is NAD 1983 or NAD 1927. If it’s NAD 1983, you can use the same text from above as a template to create your .prj file. Just change the two red areas to match what the metadata says:

PROJCS["NAD_1983_UTM_Zone_18N",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",
SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-75.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]

If it’s NAD 1927, use this template and change the red areas

PROJCS["NAD_1927_UTM_Zone_17N",GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",
SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-81.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
(Remove any line breaks that I’ve entered for readability).

Now, Go to the Add Content button and select Shapefiles… Then browse to the ArcShapes folder and add S_Fld_Haz_Ar.shp. The shapefile starts out looking like this…

…which isn’t very helpful, but just wait. If you right click on the layer in the Contents window, you can change the symbol to something with edges. Now you will be able to see the flood zone borders.

And, if you right click on the layer again and this time bring up the Properties window, you will be able to select certain attributes to show as Popup Content.

(click on the image to see full size)

When you open this dialog box, a list of all the available attributes will come up. You will be able to select which ones you want to appear in a little pop-up window whenever you click on a feature. I picked all of them. Then, in the bottom half of the box you can select one attribute that will appear whenever you hover over a feature with your mouse. I picked FLD_ZONE because this is the most important piece of information. Now, if I type the address of City Hall into the Find box and press enter, I have everything I wanted at the beginning.

There’s a “you are here” symbol, and if I mouse-over I see it’s located in Zone X (not Flood Zone). I can mouse over other areas to see where the nearest 0.2 pct annual chance flood hazard zone is. And if I click in the zone, I can get any more information that exists about it.

Knowing how to use shapefiles in ArcGIS Explorer opens up a world of information. You can watch a free basic overview of the software at http://blogs.esri.com/Support/blogs/esritrainingmatters/archive/2009/12/03/explore-arcgis-explorer-in-a-free-training-seminar.aspx

This post is inspired by a reader question. My friend Jeremy asks,

“Is there any easy way to get at GIS data of water tables, flood zones, and stuff like that? Things that people who buy houses really should look into but seldom do.”

I cut my teeth making flood zone maps so I am quite familiar with where this data lives. I revisited the FEMA Map Service Center to give him some specific advice about where to look. My plan was to determine the easiest/fastest path to the information. To my dismay, the website has not changed much since I changed jobs. Meaning, to put it bluntly: there is no easy way.

I assume most visitors to the website would want to be able to type in an address and have that address come up as a dot on a map with the flood zone layer below it. If the dot falls within a flood zone, they’d be able to tell in one glance. Well, you can get pretty close, but you can’t exactly do that. So, instead of talking about the easiest way to get flood zone information, I’ll talk about four different ways.

I attended college at George Mason University so I decided to use the city of Fairfax as my example location. Pretend you are moving there. Because I don’t want to map a private residence, pretend you are moving there because you just got elected mayor. You want to check if your new digs at City Hall are in danger of flooding.

Method 1: FEMA’s Map Viewer
https://hazards.fema.gov/wps/portal/mapviewer

Pros

• This viewer allows you to type in an address and zoom to it.
• The map that comes up shows the flood zones in that area along with a lot of other useful information.
• There’s an identify tool that allows you access even more information about the data layers.

Here’s what I got for City Hall:

Cons

• There’s no “you are here” symbol on the map. If I’m not familiar enough with the area to already know where City Hall is, I still don’t know whether it’s in a flood zone.
• There’s too many layers turned on by default, which clutters the map, making it confusing and hard to interpret at first glance.
• The legend isn’t very helpful. It doesn’t explain what the blue-hatched Zone A is, or what the purple text LOMRs are. I only know that they are Letters of Map Revision because I used to work there.

Method 2: Product Search by Address…
Box on top-left corner of main Map Service Center screen

You can enter an address and select either a Flood Map (visual image) or DFIRM (digital database) product. The Fairfax City data is at the moment only available as a DFIRM, which I will discuss later in method 4. For now, I’ll enter the address of nearby GMU campus to bring up the Flood Map of surrounding Fairfax County.

Pros

• A beautiful looking map (If I don’t say so myself) which is simple and easy to interpret, and also happens to be the authoritative flood zone representation product.
• A good legend which explains in detail what the flood zone designations mean.

Cons

• Obviously no “you are here” symbol on this static map.
• And, it’s even harder to pinpoint an exact location than in the Map Viewer, because these maps only show major roads, and most of the time use route numbers instead of road names.

It isn’t hard to see why people may feel the need to look elsewhere to determine if their prospective property is really in a flood zone. Maybe that’s why a google search for “flood zone data” brings up companies that will research this for you if you pay them. There are, however, other ways, which are free or almost free.

Method 3: FEMA’s Web Map Service (WMS) & ArcGIS Explorer
https://hazards.fema.gov/femaportal/wps/portal/NFHLWMSe

A web map service is a computer program that will query a database and deliver the results back in the form of a dynamically generated map. FEMA makes the National Flood Hazard Layer available for free through a WMS. You need to have GIS software in order to read WMS data. However, if you don’t have access to the ArcGIS Desktop package that I regularly blog about, you aren’t locked out from playing. You just need to grab a copy of ArcGIS Explorer: “GIS for Everyone” at http://www.esri.com/software/arcgis/explorer/index.html

Once you’ve installed and opened the program, press the Add Content icon on the main toolbar. Select GIS Services… and then press New Server Connection. For the server type, chose WMS, and then enter the Server address that FEMA gives on their tool description page:

http://hazards.fema.gov/wmsconnector/wmsconnector/Servlet/NFHL?

You’ll then be given a list of services to choose from. Flood Hazard Zones (General) is the main one. If you pick that, then enter your address in the Find box on the left side of the screen, the globe will whirl around until you see this.

Eureka! Now I know City Hall isn’t in a flood zone!

(I changed the Basemap to Streets)

Stay tuned, in two weeks I will post about Method 4: Using DFIRM Shapefiles. The best is yet to come.