Transform vertical datum values (e.g., NAVD88 to MLLW) at a coastal survey point using the NOAA VDatum web API
domain: vdatum.noaa.gov · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Determine the correct region code (contiguous, ak, westcoast, wgom, chesapeak_delaware, hi, prvi, as, gcnmi, etc.) for the project site
Build a GET request to vdatum.noaa.gov/vdatumweb/api/convert with s_x (longitude), s_y (latitude), s_z (elevation), region, and source frame parameters
Specify t_v_frame (e.g., MLLW, MHW, NAVD88) and t_v_geoid to select the target vertical datum and geoid model
Parse the JSON response for t_x/t_y/t_z transformed coordinates and the uncertainty field
Repeat per point or script a loop for a batch of survey shots needing tidal-to-orthometric conversion
Known gotchas
Omitting region defaults to contiguous US — coastal work outside CONUS returns wrong results unless region is set explicitly
Vertical uncertainty varies significantly by region/target frame; propagate the returned uncertainty value into project QA rather than discarding it
Horizontal frame defaults to NAD83_2011 — legacy NAD27 or NAD83(1986) data needs explicit s_h_frame or the transform assumes the wrong horizontal datum
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