Calculate

A calculate step adds a new field to the output rows whose value is computed from existing fields (or computed from the geometry itself: area, length, centroid coordinates).

Inputs

• Source layer.
• One or more new fields, each with: - A name (column header). - A type (text, integer, decimal, date, boolean). - An expression that produces the value.

Expression vocabulary

The expression dialect is a small functional language:

• Field references: bare identifiers (status, acres).
• Arithmetic: +, -, *, /, %. Integer/decimal handled like SQL.
• String concatenation: ||. (first || ' ' || last).
• Comparison + Boolean: =, !=, <, >, and, or, not.
• Conditional: case when ... then ... else ... end.
• Functions: upper(x), lower(x), length(x), round(x, n), coalesce(x, y), cast(x as decimal), now(), today().
• Geometry helpers: area_m2(), area_ft2(), length_m(), length_ft(), centroid_x(), centroid_y(), bbox_west(), ...

Output

The source rows, unchanged, plus the new fields. The geometry is unchanged.

Examples

• Acreage from polygon area: acres = area_m2() / 4046.8564.
• Full name from first and last: full_name = first || ' ' || last.
• Status flag: is_overdue = case when due < today() and status != 'closed' then true else false end.
• Buffer-ready label: display = upper(coalesce(name, 'UNKNOWN')).

When NOT to use Calculate

• One-off value transforms in a popup. Don't add a derived column just so a popup can show formatted output. The popup template can interpolate fields directly.
• Per-user values. Calculate is per-row; it doesn't know who the viewer is. Per-user logic happens at query time.

Notes

• Expressions are evaluated at materialization time. Refresh the derived layer when the source changes.
• Type coercion follows SQL rules; cast(x as integer) is the explicit form when you don't want to rely on coercion.
• NULL propagates through arithmetic: 1 + null = null. Use coalesce to default a NULL before computing.