FEMA - USA Flood Hazard Reduced Set. Is the ESRI Symbology different from FEMA Flood zone type?

Question

Hi I have noticed that ESRI has its own attribute called "ESRI Symbology" for categorizing the FEMA Flood zones in this Living atlas service: https://www.arcgis.com/home/item.html?id=2b245b7f816044d7a779a61a5844be23

I filtered the dataset for only 0.2% Annual Chance Flood Hazard, but I am still noticing Flood Zone Subtypes that are not a 0.2% Annual Chance still within the service after this filter is applied and I am wonder how ESRI + FEMA decided on this?

I did some digging an noticed that when the filter below is applied, Flood Zone Subtypes still shows areas of 1 Percent Depth Less Than 1 Foot, Area of Special Consideration and 1 Percent Drainage Area Less Than 1 Square Mile as an ESRI symbology of 0.2% Annual Chance Flood Hazard.

"esri_symbology = '0.2% Annual Chance Flood Hazard' And ZONE_SUBTY NOT IN ('0.2 PCT ANNUAL CHANCE FLOOD HAZARD IN COMBINED RIVERINE AND COASTAL ZONE', '0.2 Percent Annual Chance Flood Hazard', '0.2 Percent Annual Chance Flood Hazard Contained in Channel', '0.2 Percent Annual Chance Flood Hazard Contained in Structure', '0.2 Percent Annual Chance Flood Hazard in Coastal Zone')"

This seems incorrect to what documentation FEMA has on their site. https://www.fema.gov/glossary/flood-zones

Again I am just looking for some clarity as to how the "esri_symbology" category was curated.

Thanks!!

Michael_Dangermond_Content · Answer

Hi Nicolas! It looks like their source begins with FEMA as a point of departure and models flood risk for the rest of the USA. Their map says their data source is risk factor. For a reality check to see if a flood zone you see matches FEMA I would use the agency's own flood hazard viewer found on this page. Living Atlas team knows that there are a lot of ways to work with flood risk and we appreciate the efforts to model flood risk around the USA, but in this layer we stick close to FEMA because our customers also want to know what's legally required of them by these flood zones, at as high resolution as is practicable. Living Atlas Team also serve a layer put out by NRCS the soils people called USA SSURGO Flooding Frequency, which shows how often a soil map unit polygon is expected to flood. This has better temporal resolution than FEMA, but less spatial resolution, as detailed in this blog.

NicolasGrapsas · Answer

Hi Michael, thanks for the quick reply and detailed information, it was very helpful !

I've been able to generate the raster tiles for a given subset of texas.

While i was reviewing the results, i've noted there are some missing polygons (compared to redfin.com floodzone map)

I think the living atlas has the same problem as my version (attaching screenshot: redfin vs living atlas)

Do u know if its due a processing error or if its a fema probelm and redfin may have a different source of data ?

Michael_Dangermond_Content · Answer

Hi Nicolas, we already have a raster service of the flood zones in the Living Atlas you can just connect to and use. It's available in three resolutions, 10m, 30m, and 90m. Besides resolution you can symbolize the rasters in three ways. One is the recommended display method by FEMA, the default cartographic renderer. A second way displays only areas where flood insurance is required. The third way simply symbolizes on the FLD_ZONE field in case you want to break things out with that category.

If you really want to generate these yourself, I would recommend doing it in pieces (Alaska, Alaska in the eastern hemisphere, Hawaii, CONUS, PR/USVI, and so on). We used the dice geoprocessing tool in ArcGIS Pro with FEMA's polygons to limit the number of vertices in the input polygons to 50,000. This makes polygons a bit simpler and more compact for raster processing. We also run repair geometry in ArcGIS Pro before making rasters.

Good luck! I would check with our raster layer in the Living Atlas and see if it meets your needs. That's what we're here for is so nobody else has to do this. Also the Living Atlas raster layer is updated once a year. If you use the online layer it will magically be updated next year without you having to do a thing. Just like it was this year.