How FIA estimation works

The sampling design, the EVALID system, and the expansion factors behind every pyFIA estimate.

pyFIA implements design-based estimation following Bechtold & Patterson (2005), the foundational methodology for the FIA program. This page explains the concepts that make the estimates valid — and the one rule you must follow to keep them valid.

The sampling design

FIA uses a three-phase, nationally consistent design:

1. Phase 1 — remote sensing stratifies the landscape and identifies forested land.
2. Phase 2 — field crews measure one ground plot per ~6,000 acres of forest: species, size, condition, and site attributes.
3. Phase 3 — a subset of Phase 2 plots gets expanded measurements (crown condition, down woody material, soils).

Each ground plot is a cluster of four 24-ft-radius subplots, each with a nested 6.8-ft microplot for small trees and an optional 58.9-ft macroplot for large trees. Trees are assigned to a plot component by diameter, which determines their per-acre expansion.

The EVALID system

This is the most important concept for getting correct estimates.

FIA data is organized into evaluations, each identified by a 6-digit EVALID (SSYYTT: state, year, type). An evaluation is a complete, statistically valid set of plots and strata for one purpose — estimating area, volume, growth, and so on.

A single downloaded state contains many overlapping evaluations (different years and types). They share plots. If you estimate across more than one at a time, plots are counted repeatedly and your totals inflate — often by 10-60x.

Always clip to one evaluation before estimating. In pyFIA:

db.clip_most_recent()                  # most recent VOL evaluation
db.clip_most_recent(eval_type="GRM")   # for growth / mortality / removals
db.clip_by_evalid(442301)              # or pick an EVALID explicitly

Evaluation types

Type	Used for
EXPALL	Area estimation (most comprehensive)
EXPVOL	Volume and biomass
EXPGROW / EXPMORT / EXPREMV	Growth / mortality / removals
EXPCHNG	Change estimation

The evaluation year is a reference year for a complete estimate — not the year each plot was measured. EVALID 442301 represents Rhode Island’s 2023 conditions even though it includes plots measured across several prior years (INVYR).

Expansion factors

Estimates scale plot measurements up to the full population through a hierarchy of factors:

• TPA_UNADJ — base trees-per-acre a tree represents, by plot component.
• ADJ_FACTOR_* — per-stratum adjustments for non-sampled area, selected by tree diameter (micro / subplot / macro).
• EXPNS — the stratum expansion factor (acres represented per plot), from POP_STRATUM.

The per-acre contribution of a tree is, conceptually:

value × TPA_UNADJ × ADJ_FACTOR × EXPNS × CONDPROP_UNADJ

pyFIA applies all of these automatically — you never compute them by hand.

Land and tree classifications

Forest land (COND_STATUS_CD = 1): at least 10% stocked, ≥1 acre, ≥120 ft wide.

Timberland: forest land that is unreserved and productive (capable of ≥20 ft³/acre/year).

Growing stock: live trees ≥5” DBH meeting merchantability standards (TREECLCD = 2).

These map directly to the land_type and tree_type parameters — see Domain filtering.

Volume types

Variable	Description
VOLCFNET	Net cubic-foot volume (defects deducted) — the growing-stock default
VOLCFGRS	Gross cubic-foot volume (no deductions)
VOLCFSND	Sound cubic-foot volume (excludes rotten/missing cull)
VOLBFNET	Net board-foot sawlog volume

Growth, removals, and mortality (GRM)

GRM estimates come from remeasurement, stored in dedicated tables:

• TREE_GRM_COMPONENT — each tree’s fate component (SURVIVOR, MORTALITY, CUT, INGROWTH).
• TREE_GRM_MIDPT — measurements at the remeasurement midpoint, for annualization.

Because GRM uses different evaluations, remember eval_type="GRM" when clipping.

Common grouping codes

Ownership (OWNGRPCD)

10 = National Forest · 20 = Other Federal · 30 = State/Local · 40 = Private

Site productivity (SITECLCD)

1 = 225+ · 2 = 165-224 · 3 = 120-164 · 4 = 85-119 · 5 = 50-84 · 6 = 20-49 · 7 = 0-19 (cu ft/ac/yr)

Tree status (STATUSCD)

1 = Live · 2 = Dead · 3 = Removed (cut)

Further reading

• Understanding variance — how pyFIA computes standard errors
• Bechtold, W.A. & Patterson, P.L. (2005). The Enhanced FIA Program — National Sampling Design and Estimation Procedures. Gen. Tech. Rep. SRS-80. DOI: 10.2737/SRS-GTR-80