Fire Hydrants: the New Geodetic Markers?

Traditional surveying involved distance and angle measurements from a network of geodetic markers with known coordinates. Even single-baseline differential GNSS relied heavily on such a network of reference points. With recent advances in GNSS processing, ground markers are effectively being replaced by precise satellite orbits, enabling positioning methodologies such as precise point positioning (PPP). Therefore, investments made by geodetic agencies to maintain and densify classical geodetic networks have been considerably reduced, even though users still rely on such markers for their day-to-day operations. This post proposes to tap into existing infrastructures as an alternative to geodetic markers.


The expansion of RTK networks is yet another factor against investing in geodetic markers: users turn on their GNSS receivers and obtain almost instantaneously cm-level accuracies. Some organizations have however recognized the need for land surveyors to validate network RTK surveys on the fly, thereby relying on the precise coordinates of ground markers. While there is no doubt that validation of GNSS surveys must be accomplished, I believe that maintaining geodetic markers is simply a costly and time-consuming endeavor.


While taking a walk in my neighborhood on a frigid winter evening, I noticed that fire hydrants were roughly 200 m apart, standing tall above snow banks. A quick search on the City of Ottawa website revealed that the city maintains and tests, on a yearly basis, over 22,000 fire hydrants on its territory. It occurred to me that performing a RTK survey of those entities to determine their coordinates with cm-level accuracies would not only make a nice summer job for geomatics students, but could become a very useful tool for land surveyors. Consolidating the position of all fire hydrants in Canada in a centralized database could become a powerful asset for anyone in the geolocation business.

Fire hydrant


For the rest of my walk that evening, I made a game of locating all fire hydrants. It reminded me of the thrill I first had when I discovered that there were geodetic markers in the sidewalk. It also brought back memories of the challenges associated with setting up a tripod over fire hydrants...


Of course, the location of fire hydrants was not selected with GNSS surveys in mind, but neither was the location of classical geodetic markers.

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Comments: 6
  • #1

    Landon (Monday, 15 February 2016 21:04)

    Hey Simon,

    You should check this out:

    Was even granted patent!

  • #2

    Simon Banville (Thursday, 18 February 2016 20:14)

    Thanks for the interesting link, Landon! I just can't believe it:

    "Ask the municipality for a copy of their Licensing Agreement that allows for the fire hydrants to be used in accordance with the applicable Hydrant Monument patent. If they have no Licensing Agreement then you must immediately stop using the hydrants. Any infringement is considered a serious matter."

    Land surveyors have been using fire hydrants as control monuments for decades. My idea was to store the coordinates into a central database to replace the aging conventional network. Not only Hydmon Inc. does not provide coordinates, they prevent us from using the hydrants as monuments without paying them royalties... non-sense!

  • #3

    Mundozone (Thursday, 05 May 2016 02:28)

    Good idea. Great article!

  • #4

    Miquel Garcia (Monday, 16 May 2016 01:34)

    Just one question, how do you define the HRP (Hydrant Reference Point)?
    Nice and very interesting idea!

  • #5

    Vin (Monday, 16 May 2016 10:36)

    Surveyor discussion of the patents:

    A post on page 2 links to this lawsuit, Hydmon Inc. v. City of Orillia, settled out of court:

  • #6

    Simon Banville (Tuesday, 17 May 2016 21:28)

    Thanks Vin for these interesting links!

    Several comments stated that the top nut was not a stable reference point: I can easily picture the height of the nut changing after the hydrant is used. Are horizontal displacements an issue as well? For network-RTK survey validation, a couple of cm wouldn't be a major concern...