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The FAA has a Digital Obstacle File (DOF) which reports on tall objects that would be important for pilots which is updated every 56 days. https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dof/

They also have a Daily DOF which purports to do the same thing but is updated daily. https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dailydof/

Why have both?

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There’s very different use cases for the information.

If I am putting up a 200 foot meteorology tower in a city, this is of no concern to commercial, military, or general aviation operators. So it will be placed in the DOF right away, but since it doesn’t affect flight in the normal IFR or VFR environment, it doesn’t have to be propagated to all pilots right away. But there may be some special operators--mosquito spraying, ag application, etc.--that may want to check the DOF before a specific sortie, so the daily is available for them. Organizations that work with this data to create, or consume this data can get a an "OE/AAA" online account.

A taller tower that penetrates routine use airspace or even a shorter tower that approaches the obstacle clearance gradient near an airport is another story completely. In these cases, the proposed tower is coordinated with the FAA OEG (Obstruction Evaluation Group) and goes through a process called OE/AAA. The obstruction goes through an analysis because VFR charts and instrument procedures all need to be updated. Oddly, the notice required by the FAA is only 45 days before start of construction, so they're assuming it will take you a few weeks to penetrate airspace. If needed, new obstacles that do penetrate airspace and are not yet charted will be addressed by NOTAM.

Also, any tower over 2,000 feet AGL falls in a different category. Below 2,000 feet you are primarily "coordinating" with the FAA. But anything over 2,000 feet is considered a "potentially inefficient use of airspace" which triggers a required approval process by the FAA, similar to an impact statement. There are no buildings over 1,500' in the US, 20 radio towers exactly at 2,000', and only 4 taller; so this isn't a call the FAA gets often.

The FAR that covers this is Part 77. 14 CFR 77.9 states:

Any person/organization who intends to sponsor any of the following construction or alterations must notify the Administrator of the FAA:

  • Any construction or alteration exceeding 200 ft above ground level
  • Any construction or alteration
    • within 20,000 ft of a public use or military airport which exceeds a 100:1 surface from any point on the runway of each airport with at least one runway more than 3,200 ft.
    • within 10,000 ft of a public use or military airport which exceeds a 50:1 surface from any point on the runway of each airport with its longest runway no more than 3,200 ft.
    • within 5,000 ft of a public use heliport which exceeds a 25:1 surface
  • Any highway, railroad or other traverse way whose prescribed adjusted height would exceed that above noted standards
  • When requested by the FAA
  • Any construction or alteration located on a public use airport or heliport regardless of height or location

As is often the case on Stack Exchange, you now officially have too much information.

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    $\begingroup$ I don't really see there are 2 different use cases. I can see why someone might not care to get the new daily file everyday but why would anyone forego the previous day's daily file to get the file that could be up to 56 days old? $\endgroup$ Jan 26, 2023 at 18:20
  • $\begingroup$ @DeanMacGregor Remember that these files are not intended for use in flight planning. Published and NOTAMs are authoritative, and in Parts 91, 121, 135, etc we are required to fly using them. We may find the DOF data interesting but we are not the intended audience. There are very foreseeable cases where a data consumer needs the exact DOF version that corresponds to an exact version of published charts they are using, so they want to be able to download the file upon which those charts were based. (cont...) $\endgroup$
    – Max R
    Jan 27, 2023 at 7:44
  • $\begingroup$ (...cont) But 99% of the time, we're not supposed to be doing anything with the DDOFs, we're supposed to be using NOTAMs, the only approved way for us to consume the information. You see the 56 day file as being "outdated" but many use cases see it as "matching the current charting." Part 77 requires the files to be made public, and so they are posted to a website, but that does not mean that aviation operators are the intended audience for those files. $\endgroup$
    – Max R
    Jan 27, 2023 at 7:46
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I was curious myself, so I actually asked the FAA. This is the response I received:

I received your inquiry asking about the difference between the DOF (Digital Obstacle File) and the DDOF (Daily Digital Obstacle File). You are correct that the two products provide generally the same data, one on a daily basis, the other every 56 days. Due to the fact that our stakeholders have a variety of needs, where some need that data to match other products, such as charting, which provides updates on a 56 day cycle, while others need the data changes much more frequently, the two products may have entirely different users for much different reasons. We want to provide the obstacle data for those stakeholders who have different needs.

I hope this makes sense and provides you with an explanation for the similar but very different products and their uses. Should you have any other questions, please don't hesitate to contact me.

Personally I find "generally the same" and "similar" to be completely unhelpful. It would make perfect sense to supplement a 56 day database with a daily update of what has changed since the last release, but if the daily data includes ALL obstacles there is really no need for the 56. Just go get the latest daily data whenever you want an update.

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    $\begingroup$ Thanks for reaching out to them! As was mentioned in their response, “some need that data to match other products, such as charting…” I mentioned this in my answer as well. Remember that the use case for these files is not pilots but people that are correlating data for various uses. Over the years I have downloaded DOF files maybe 5-ish times, and in each of these projects, it was important that I had the data that matched the currently released charts and procedures exactly. Most of the use cases others have for the data I am aware of are similar and require the 56 day DOFs. $\endgroup$
    – Max R
    Jan 28, 2023 at 4:35
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The 56 day file is used to update charts and published procedures (which are also on a 28/56 day cycle). Obstacles that are not yet incorporated in the 56day publications must get a NOTAM.

It is one database. The two object files are snapshots. There are also change lists for each. The change lists contain additions, removals, and updates to accuracy since the last snapshot(one day or 8 weeks of change). The 56day file and 56day change list are particularly useful to keep the many teams in many locations around the country working on many products all synchronized and avoid mistakes with exactly which day's data is being used and which data [changes] need special attention.

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  • $\begingroup$ So does the daily update ONLY contain obstacles that are NOT listed in the 56 day? $\endgroup$ Jan 28, 2023 at 19:27
  • $\begingroup$ @MichaelHall It is one database the two object files are snapshots. $\endgroup$
    – Max Power
    Jan 29, 2023 at 3:53

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