docs(ogc): record the null measuring-point height default

water_well_summary, depth_to_water_trend_wells and
water_elevation_wells layers all substitute zero for a missing
measuring-point height, treating the measuring point as ground
surface. That has been in the SQL since those views were written
(COALESCE(measuring_point_height, 0)), but nothing said whether zero
was the intended default or an accident. BDMS-974 surfaced the
question and deferred it here rather than change the views on a guess.

Cris Morton and Ethan Mamer confirmed on 2026-08-26 that zero (ground
surface) is the right default, so the views are correct as written and
there is no code change to make. What was missing was the record: the
next person to read that substitution would ask the same question, and
without an answer on file the safe-looking move is to exclude those
rows, which would silently drop wells from three published layers.

The doc keeps the consequence visible rather than acting on it. Wells
with no recorded measuring point are less certain than the rest and
nothing in the output says so. Adding a flag column would touch three
view definitions and the field descriptions for a subset nobody has
sized yet, so the decision records the condition for revisiting it
instead.

Refs BDMS-980

Author: ksmuczynski

docs/measuring-point-height-null-handling.md

@@ -0,0 +1,26 @@
+# Measuring-point height: null handling
+
+**Status:** Accepted
+**Date:** 2026-08-26
+**Decided by:** Cris Morton + Ethan Mamer
+**Recorded by:** Kelsey Smuczynski
+
+## Context
+
+Three OGC layers compute depth-to-water and water-elevation values from measuring-point height:
+
+- `water_well_summary`
+- `depth_to_water_trend_wells`
+- `water_elevation_wells`
+
+Each uses `COALESCE(measuring_point_height, 0)` in the calculation. When `measuring_point_height` is null, this treats the measuring point as ground surface level (0), rather than excluding the record or flagging it for review.
+
+## Decision
+
+A missing or null measuring-point height defaults to zero (ground surface level) in these calculations. Cris Morton confirmed this is the correct default on 2026-08-26.
+
+## Consequences
+
+- Depth-to-water and water-elevation values for wells with no recorded measuring-point height are calculated as if the measurement were taken at ground surface. These values carry more uncertainty than wells with a known measuring point, but nothing in the output currently distinguishes them.
+- If a future review determines this default introduces meaningful error for a subset of wells (for example, wells with a well casing or stickup that's unusually tall), revisit this decision and consider flagging affected records instead of silently defaulting to zero.
+- Any new analytic layer that calculates depth-to-water or water-elevation from measuring-point height should follow this same convention, or explicitly document why it doesn't.