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Finding Floodplain GEOJSONs


    Earlier this year I participated in the Houston Hackathon. I led a group working on an idea I created to have a front-end only website that would tell you when you entered or exited a floodplain in Houston, Texas while you were driving or riding a bicycle. You can find the code for that project on GitHub and the live site at . As part of that project, I needed data files for the spatial outline of floodplains in Houston, Texas.

    Despite having run open data and open code sites at NASA, I still stumbled a bit finding the right open data for this task due to some incorrect assumptions. Those assumptions led to several hours of wasted time over a couple weekends before the hackathon. I am writing up these lessons learned in this blog post in case they might help others.

    What Data Did I Ideally Want?

    I ideally wanted floodplain data that was:

    • In GEOJSON format as that's easy to work with on the web with JavaScript. In a pinch, I'd accept a shapefile, which is a proprietary data format that can be relatively easily converted into GEOJSON.
    • An extend limited to the outline of either the city of Houston or Harris County, so I wouldn't have to combine files and then reshape them.
    • Data with the most recent published FEMA designated floodplains. Floodplain boundaries change over time.

    I could budge on all of these to some degree, but I wholely expected to be able to find data that met these criteria.

    Where I Recommend You Go After Wasting My Time

    Go to FEMA for floodplains. They likely have the data you want, but you might have to dig quite a bit to find it as they provide differently packaged data in different places!

    Specific Webpages that Were Useful for My Needs...

    Step by Step How to Use FEMA National Flood Hazard Layer County-Clipped GIS Data Selector

    1. Follow the link in the second bullet above
    2. Put in an address within the county you want.
    3. Ignore the pop-ups on the screen! They won't get you a county shapped GIS export!
    4. CLick anywhere on the map itself
    5. On the new pop-up that appears where you just clicked not the original one, click "Download county GIS data".
    6. Wait on your browser to download the zip file.
    7. Unzip the zip file. You'll get a folder with the same name.
    8. Within that folder, look for a file called something like "S_FLD_HAZ_AR.shp", that's the shapefile with the floodplain polygons specific to the county where you clicked in step 4.
    9. To convert that shape file into a GEOJSON, see this jupyter notebook that goes through the conversion process.

    What Open Data Sites were Tempting But Ultimately Did Not Have the Right Data

    After initially looking at the FEMA website and not finding floodplain files constrained to Houston city or Harris County outlines, I decided to look at municipal and county sources of data.

    My assumption at the time was:

    1. The FEMA website did not have the data already limited to a city or county extent (this was wrong)
    2. and it would be faster to find this data on county and city open data sites than it would be to write the scripts to reorganize the FEMA data into a spatial extent I wanted. (this was also wrong)

    Houston Galveston Area Council Open Data

    Houston Galveston Area Council is a regional planning organization. I've previously found land cover and river basin open data there. I know they provide some information on floodplain management as well, so they seemed like a good place to look.

    What I found:

    Why it Didn't Work:

    The file generation process required before allowing you to download the data in GEOJSON format never finishes! I waited around for this and redid it a couple of different ways, still no luck. Additionally, they only have data as of 2015.

    Haris County Open Data

    Harris County GIS Open Data Portal has a variety of traditional local government open data types at the county level. Houston is wholely within Harris County.

    What I found:

    • If you search for "floodplain" in their search interface, you get zero results.
    • However, if you look at this broader open data page not specific to GIS data, you get more things related to flooding. However, none of them are floodplain boundaries.
    • For user friendly options for seeing the floodplains, there is this web map experience for floodplain boundaries, but you can not get data files out of it.
    • Digging a little further into the ArcGIS site, I found there is at some sort of 2019 era FEMA floodplain boundaries available even if they do not come back in search results. Unfortunately, every time I tried to download them it timed out. It seems without authentication there is a rather short time out window to make an API call. I could never get the entire dataset to download.

    Why it Didn't Work:

    Although the county websites definitely have floodplain polygons, they do not make it possible to download the raw files.

    City of Houston

    The city of Houston's open data portal has seen a noticable decrease in activity and quality in the last couple of years. Another person was selected to lead it at once point, but then nothing really changed in terms of what is seen by users.

    If you follow the top Google search results, you get directed to which may not load for you without an error message as their https certificate is out of date and has been for years. You'll have to click through to accept the risk to actually land on their page.

    What I found:

    • If you click on the button for "flood hazards", there is one dataset titled "Other County Flood Zones".
    • If you instead use the search bar and put in "flood", you'll get back 4 datasets as of October 2022. This data was last updated in 2017 though, so it isn't the recent floodplain maps.

    Why it Didn't Work:

    The City of Houston open data portal has not gotten a lot of attention in recent years. There is some data, but much of it is up to date.

    Lessons learned

    FEMA Provides the Same Data In Many Different Ways at Different Webpages

    The first lesson learned is that I too easily assumed that the data I found on FEMA's website is the only version of that data. Whenever an organization has the authoritative data and many different types of users and use cases, they tend to provide it in different ways to meet all those use cases. I should have spend more time digging around the FEMA site, but I got frustrated and left assuming I had seen all the ways the data was supplied to different users.

    The Importance of Thinking About How Providers Think About The Data You Want

    FEMA floodplain boundaries have insurance, legal, and political implications. That's likely a forcing factor that will push city and county government to provide ways to access the data in informative means like web maps, but not necessarily the raw data, which they don't want to be responsible for. This was seen in the Harris County open data.

    Open Data is Less of a Focus in Government

    Lastly, open data is decreasing as a government initiative. You see this at the federal, state, county, and city level.

    The is also a whole history of rising interest in open data in government, especially during the Obama presidency, that is definitely worth reading about it. More recently, there has been a trend for these efforts to get less attention in government. You see this in drop in quality of the City of Houston's open data. I saw it in some parts of NASA, but not all! Part of this is due to trend-driven IT in government. Part of it is due to what Congress tells agencies to work on. There are lots of interesting forcing factors that others have written about better than I.

    As a result of this trend, there's a lot of older, not-update data that exists from 2009-2013 period that can tempt you to looking for more data that you suspect must exist, but that hasn't been updated for the past 0-7 years. This is seen in the City of Houston, Harris County, and Galveston Houston Area Council's open data sites.


    If you want floodplain data, go to the authoritative source, FEMA, and keep diging. State, regional, county, and city sources are likely to waste your time.