tidemark

Satellite altimetry against Maine’s tide gauges.

Read the Background
Reproduce the Work

Two ways to establish an elevation.

TodayA crew, when one can come

A new gauge reads the water from its first hour, yet none of its readings reach a forecast until a crew surveys the sensor’s elevation with high-precision equipment, one site at a time.

This project asksA satellite, every 21 days

SWOT crosses the same coast on a fixed repeat whether anyone schedules it or not, and every pass over a gauge reads the same water from orbit, so enough passes may settle the sensor’s elevation within the five centimeters a survey is designed to hold.

hdz
  • h

    Above the datum

    The water’s height above the datum, which the pass reads in the instant its track comes over the dock and never between one crossing and the next.

  • d

    Down to the water

    The distance from the sensor down to the water, which is the whole of what the gauge itself reports and the reason its series cannot be read as a height.

  • z

    The sum nobody has

    The sensor’s own height above the datum, which is the two lengths above added together and the figure a survey crew would otherwise have to come out and supply.

  • What a pass leaves

    Every place the pass came over keeps the height it measured there, standing up from the datum at even intervals, with the gauge the last of them.

  • The pass overhead

    Two swaths read the surface either side of the track, and an altimeter pointed straight down the track measures the gap at nadir between them.

tidemark

Tidemark ​

Satellite altimetry against Maine's tide gauges.

Installing a new tide gauge on the Maine coast waits on a survey. The gauge itself is a downward-looking sensor that measures the distance to the water, and converting that distance into a water surface elevation needs the exact elevation of the sensor, which a crew establishes on site with high-precision equipment. The network of low-cost gauges feeding Maine's coastal flood forecasts therefore grows only as fast as those visits are scheduled.

The Surface Water and Ocean Topography satellite measures sea level near the coast from orbit. It measures a given stretch of shore only when it passes overhead, so it cannot replace a gauge for forecasting, but every pass over an existing gauge is a second reading of the same water surface. Tidemark compares those readings across Maine's gauge network to find out whether enough passes can establish a sensor's elevation as well as a survey does.

The Network ​

Each gauge the station catalog carries on the Maine coast takes a ring for every pass whose swath covers it, as the passes come over one at a time through the 21-day cycle. That holds for every anchor and target, and for Wells, which NOAA has retired. A gauge that sits in the gap of every pass takes a dashed halo instead. Beneath them, the water and the land take close tints of one blue, stepping down with depth and up with height. mise draw redraws the map from the pass schedule, and the schedule's map zooms in on any one gauge.

Anchor
Target
Ring per pass
In the gap of every pass
Pass reaching a gauge
Every gauge on the coast31 places · 24 under a swath

One cycle of 21 daysEvery 21 days

Passes9
4 north, 5 south

Most16 of 16 · anchors 3 of 3

Port Clyde, Lindsey Brook, Apprenticeshop, and Rockland Public LandingMost covered · 4 rings

First pass  7 · north · day  1

This cycle58 / 58 rings
Day 1Day 8Day 15Day 21735214242270285313520548
Commit47945eaDataschedule.jsonUnreleased1.2.0

Data from

  • 1Statistics Canada
  • 2U.S. Census Bureau
  • 3GMRI, NERACOOS
  • 4USGS
  • 5NOAA CO-OPS
  • 6SWOT, PO.DAAC
  • 7GMRT

First Soundings ​

A crew coming into unfamiliar water takes its first soundings to find a way through, and a reader new to the project takes these sections first. They set out why a gauge waits on a survey, what data the project keeps, what it computes from them, and how to run it, each opening on the page named with it, whose feature stands in the window beside the staff.

  1. BackgroundWhy a gauge’s reading is unusable until its sensor has been surveyed, and what a satellite could changeThe Unsurveyed Gauge
  2. DataWhere each kind of data the comparison reads comes from, how the project fetches and keeps it, and what each column it keeps holdsInside the Station Catalog
  3. MethodsWhat the project computes from the data, the pass schedule included, and why each step is built the way it isThe Analysis Graph
  4. ReproduceHow a reader gets from an empty directory to a machine that fetches the data and runs the comparison, and what a change passes before it reaches the main branchFrom a Fresh Clone

Datum · read upward

tidemark.fyi/background/network5 min

h d z

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

Choose a crew or a pass, and drag the water by its grip

The Unsurveyed Gauge
Commit47945eaDatastations.jsonUnreleased1.2.0

The Data ​

  • Each gauge's series

    Every water level a gauge has published, with its quality flags.

    Public ERDDAPPublic API

  • The SWOT passes

    The sea surface heights of every SWOT pass within reach of a gauge.

    Free account

  • The station catalog

    Where each gauge sits, who runs it, and what measures the water there.

    Shipped

  • The pass schedule

    Which passes cover each gauge, when, and how far across the swath.

    Shipped

  • The stakeholder

    The surveyed elevation of each sensor that has one, and any series from before a gauge joined the ERDDAP.

    By hand

The Data overview sets out each kind of data the project keeps, the side of the comparison it serves, and how a clone comes to hold it, and a page of its own follows each back to the services that publish it.

Commit47945eaDatastations.jsonUnreleased1.2.0

The Team ​

  1. 1Statistics Canada 2021 Provinces/Territories, Cartographic Boundary File, 2021 Census https://www12.statcan.gc.ca/census-recensement/2021/geo/sip-pis/boundary-limites/files-fichiers/lpr_000b21a_e.zip↩︎

  2. 2U.S. Census Bureau 2024 2024 Cartographic Boundary File (SHP), State and Equivalent Entities for United States, 1:500,000 https://www2.census.gov/geo/tiger/GENZ2024/shp/cb_2024_us_state_500k.zip↩︎

  3. 3Gulf of Maine Research Institute & Brown University 2026 NERACOOS ERDDAP Northeastern Regional Association of Coastal Ocean Observing Systems. https://data.neracoos.org/erddap/↩︎

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

  5. 5NOAA CO-OPS 2026 CO-OPS Data Retrieval API and Metadata API https://tidesandcurrents.noaa.gov/web_services_info.html↩︎

  6. 6SWOT 2025 SWOT Level 2 KaRIn Low Rate Sea Surface Height Data Product Version D. NASA Physical Oceanography Distributed Active Archive Center. https://doi.org/10.5067/SWOT-SSH-D↩︎

  7. 7Ryan et al. 2009 Global Multi-Resolution Topography Synthesis https://doi.org/10.1029/2008GC002332↩︎