The Unsurveyed Gauge ​

A coastal flood forecast reaches a Maine town only where a tide gauge can check it, and a new gauge tells a forecaster nothing until a survey crew has measured how high its sensor sits. Tidemark asks whether a satellite can supply that height instead. This page sets out what the survey is for and what it achieves, drawing on the 2026 account by the team who installed Maine's low-cost gauge network⁠1⁠.

Forecast Coverage ​

The National Weather Service offices in Gray and Caribou model the total water level along the whole Maine coast, yet they release a forecast only where a gauge or a human observer can validate the model. That left 16 forecast locations in the state when the network's installation began in late 2022. Coverage thins toward the rural east, which shows in how many Coastal Flood Warnings a county received between January 2006 and November 2025:

RegionCountiesWarnings per County
SouthernYork and Cumberland78
MidcoastSagadahoc, Lincoln, Knox, and Waldo45
DowneastHancock and Washington19

A gauge a few towns away is a poor stand-in, because the shape of the coast limits how far one reading applies. In the storm of January 2024, the new gauge at Machias recorded a peak 30 centimeters above the nearest federal gauge, 20 kilometers to the east. That difference put the water over the Route 1 dike, which flood planning based on the distant gauge had assumed would hold.

Two Kinds of Gauge ​

NOAA's National Water Level Observation Network (NWLON) is the standard every other gauge is judged against. Its stations are built to track sea level rise to the millimeter, which makes them costly, and Maine has five of them. The low-cost gauges that Hohonu and Greenstream make give up that precision for a price a town can carry.

GaugeBuilt ForInstallationYearly Upkeep
An NWLON stationLong-term sea level trends and tidal datums$75,000 to more than $1,000,000$25,000 to $50,000
A low-cost gaugeFlood monitoring, forecasting, and local flood thresholdsA sensor costing hundreds of dollars, beside the siting, the survey, and the staff time around itFor a Hohonu gauge, a subscription under which the maker replaces or upgrades a failing sensor

The network's own team states that a low-cost series cannot measure long-term sea level change and may not be reliable enough for a high-consequence operational use. It runs the QARTOD quality tests and publishes their flags beside the readings of every gauge but the one at Scarborough on the NERACOOS ERDDAP⁠2⁠ rather than dropping what they catch. Inside the Station Catalog describes how the project reads those gauges.

Distance and Survey ​

A low-cost gauge is an ultrasonic or a radar sensor fixed above the water and looking down, and what it reports is the distance from the sensor down to the surface. That distance becomes a water surface elevation, a height that can be compared with a road, a dike, or a forecast, only once the elevation of the sensor itself is known.

The network's crews establish that elevation by survey, following the protocol the Southeast Coastal Ocean Observing Regional Association (SECOORA) wrote for gauges of this kind:

  1. Set GNSS receivers, which fix a position from navigation satellites such as GPS, over the site.
  2. Measure the sensor against a benchmark, either an established one of the National Spatial Reference System or a new one fixed by a long static measurement processed afterward.
  3. Tie in three control points on stable ground around the site.

The result places the sensor on the North American Vertical Datum of 1988 (NAVD 88), the reference surveyors use for heights on land, and the protocol is designed to hold the total vertical uncertainty under five centimeters. The paper does not report the uncertainty its own surveys reached, so five centimeters is the design goal an estimate from orbit has to meet rather than a measured figure.

That surveyed elevation is the number Tidemark estimates for each target without a survey, and the stakeholder holds the elevations the network's surveys established, for which the station catalog keeps a column.

z
The elevation of the sensor
a survey fixes
h
The height of the water
the satellite reads
d
The distance down to the water
the gauge reads

Only one of the three has to be surveyed. A gauge reports d from the day it is installed and a satellite pass supplies h, which leaves z as the one length this page is about.

Two Datums in Play ​

A surveyed gauge reports heights above NAVD 88, whereas the Weather Service's guidance gives the water level above mean lower low water (MLLW), one of the tidal datums NOAA defines, which averages the lower of each day's two low tides. Correcting a forecast with a gauge therefore needs the offset between the two datums at that spot. Where a gauge stays wet through the whole tide, the team computed the offset from the gauge's own series with NOAA's Tidal Analysis Datum Calculator. At Machias, where the site drains at low tide, they took it from NOAA's VDatum model instead⁠1⁠.

The two sources do not always agree. At the Bath gauge, on a tidal river, the observation-based offset was 32 centimeters less than the modeled VDatum offset, and the authors report on page 10 that it remains uncertain whether river influence, insufficient model resolution, or some other cause explains the discrepancy⁠1⁠. We take that as a caution for our own work, since a height moved from one datum to another carries whatever error the offset does.

The Height From Orbit ​

The Surface Water and Ocean Topography (SWOT) satellite measures the height of the water surface from orbit, against a reference of its own that has to be brought onto NAVD 88 before anything is compared. Where one of its passes crosses a gauge, the satellite's height for that patch of water and the gauge's distance down to it describe one surface at one moment, once the two are matched in time and place, so the two added together point at the sensor's elevation. The sketch below draws no measurement, and marks the water’s height above the datum h, the gauge’s distance down to the surface d, and the sensor’s elevation z the two add up to, so that h+d=zh + d = z.

h d z

The band around the sensor is the five centimeters a survey is designed to hold its height inside.

A single pass is noisy, and a pass that also crosses an anchor, a NOAA station⁠3⁠ or a USGS gauge⁠4⁠ whose elevation an agency has already established, shows how far that pass's own height is off. Tidemark tests whether enough passes, each checked against the anchors it crosses, settle on an answer inside the survey's five centimeters, as the front page sets out in brief, and Data says where each kind of data the comparison reads comes from.

h d z

Where a pass crosses an anchor

Its vertical reference is already established, so the sensor’s elevation z is known. The pass’s h and the gauge’s d have to add up to it, and whatever the two miss by shows how far that pass’s own height is off there.

z

Where a gauge meets its survey

Where a target has a survey, it is held aside as the judge, and an answer from the passes counts only inside the band that survey is designed to hold.

Commit47945eaDatastations.jsonUnreleased1.2.0

  1. 1Baranes et al. 2026 Supporting Coastal Flood Forecasting and Community-Led Adaptation With Tide Gauges, Community Science and Youth Engagement Frontiers in Climate 8: 1757212. https://doi.org/10.3389/fclim.2026.1757212↩︎ ↩︎ ↩︎

  2. 2Gulf of Maine Research Institute & Brown University 2026 NERACOOS ERDDAP Northeastern Regional Association of Coastal Ocean Observing Systems. https://data.neracoos.org/erddap/↩︎

  3. 3NOAA CO-OPS 2026 CO-OPS Data Retrieval API and Metadata API https://tidesandcurrents.noaa.gov/web_services_info.html↩︎

  4. 4United States Geological Survey 1994 USGS Water Data for the Nation https://doi.org/10.5066/F7P55KJN↩︎