How ALTO is made

Sources

ALTO combines tree records from their publishers with airborne LiDAR surveys and aerial photographs. Each source record keeps its original identifier and position in the tree’s full record. ALTO crops, resamples, reprojects and combines these data, matches records to LiDAR crowns and adds its own estimates. The organisations that publish the data have not reviewed or endorsed ALTO’s estimates.

Species names come only from source records. Tree pages credit and link each register that lists a tree.

• Sources and creditsEvery dataset, its licence and its credit
• What ALTO changesSources and credits

Crowns from the 2024 LiDAR survey

Release 5 redrew Auckland’s canopy from the 2024 LiDAR survey and aerial NDVI (normalised difference vegetation index). Each LiDAR crown 3 m or taller is one marker on the map. Separate layers show canopy 2–3 m tall and vegetation below 2 m, and those layers do not contribute to benefit totals.

Crowns are not merged beyond 400 m²; a larger tree can be split into several crowns. A crown outline can still split one tree or join several: large spreading and multi-stem trees can be split or partly trimmed.

Release 6 draws each crown with a smoothed outline around the same cells; the crown’s area, height and every estimate keep the cell area. Release 6 also estimates a growth form for each crown from its LiDAR returns, its surroundings and the 2024 aerial photograph. A growth form is shown only where calls of its kind agreed with the reference labels often enough when tested; otherwise the crown reads “growth form not yet determined”. A growth form is a coarse class. The labels made from imagery, used in training and in the checks, come from a machine-learning image-interpretation model in one pass; no independent second labeller has checked them.

Each crown carries the measures in Table 2. The tree page shows each with its source, date and margin; Species maxima ranks height, crown spread and crown area.

• How crowns are drawnRelease 5
• The 400 m² merge limitRelease 5
• Growth formRelease 6
• The growth-form auditRelease 6
• Smoothed outlinesRelease 6
• Palm checkRelease 6.2
• Height and record checksRelease 6.3

Linking records to crowns

Records are linked to crowns using the actual crown boundary. Where a record lies outside every crown, nearby crowns are scored on the distance from the record to the crown’s edge, whether the crown’s height and area suit the recorded taxon, conflicts with other records’ taxa on the same crown, and how many records a crown of that size usually carries. A record is linked only when its crown is clearly better than every alternative, including no crown at all. Records that cannot be linked with confidence go to review.

A link is spatial evidence that a record describes the tree on that crown. One crown may contain several trunks or trees, and a crown drawn from a record is a proposal until it is confirmed. Unverified Notable Trees schedule positions stay at their source locations and are excluded from automatic crown matching. “No crown linked” does not mean a tree is absent.

Every marker on the map, and every earlier LiDAR detection kept as a record, has a permanent tree number, such as TMCTT6X4, and a permanent link. An alias table resolves every tree number, record ID and crown key ALTO has published to the tree that carries it now, or to the reason it carries none.

• Tree identity and protected-tree positionsRelease 5
• What a link meansRelease 5.1
• How records were linkedRelease 5.2
• Records that stay without a crownRelease 5.2
• Tree numbersRelease 6.1
• Older IDsRelease 6.1

Estimates and their ranges

Crown heights, areas and spreads are measured from the crowns of the 2024 LiDAR survey. Every value ALTO derives from them with a model is marked Estimate and, where the method gives one, shown with its range.

• Trunk diameter. The diameter at 1.4 m above the ground (on a slope, from mid-slope), the New Zealand convention; earlier methods pages say 1.3 m above the uphill ground, and no estimate changed. Each crown holds two model estimates, one from crown size and height and one from the benefit equations; the tree page shows their median as the central estimate, with the two models as its range. A trunk measured at a reviewed KYTE ground visit replaces both.
• Root space. Root extents are scenarios drawn from the trunk estimate and its range, with known paving where available. The root extent itself is not measured.
• Benefits. ALTO benefit scenario: simplified equations adapted from published i-Tree Eco components (USDA Forest Service and cooperators), with stated unit values; they are not run in i-Tree Eco. From release 6.3 the benefit value is stormwater plus carbon; PM2.5 removal and the cooling index are kept in the full record and not published. Benefit and trunk estimates are not independently validated; they remain sensitivity scenarios.
• Height class and crown replacement time. From release 6.3, the crown’s height as a share of a reference height for its growth form, and the years a newly planted tree would take to grow a crown of its area, in bands; crowns under 5 m tall, or whose height is not plausible, are not classed.
• Growth form. Marked Estimate, and shown only under the test described under Crowns from the 2024 LiDAR survey.
• Forecasts. Height scenarios start at the current height and reuse an earlier fitted model, whose transfer to the release 5 redraw has not yet been validated.

• Dimensions and estimatesRelease 5
• Consistent records and estimatesRelease 5.1
• Trunk diameterRelease 6
• Benefit estimatesRelease 6
• Service estimatesRelease 6.2
• Benefit valueRelease 6.3
• Height class and crown replacement timeRelease 6.3
• LimitsRelease 6

Reviews and corrections

Reviewer decisions about trees, crowns and records are kept in an append-only corrections ledger. Each release applies them, or records them with the trees they name, and publishes a receipt of every outcome. Each tree page lists the decisions that name the tree, its records or its crown.

Records with competing crowns, with nothing of tree height nearby or with a position that cannot be matched go to the researcher workbench, and every crown drawn from a record awaits confirmation there. Confirmations reach the map in a release.

Ground visits are recorded in KYTE: photographs, notes and measurements made at the tree. A visit appears on the tree’s page after a reviewer accepts it. Email the Better Places Lab (t.welch@auckland.ac.nz) to correct a record, with the tree number and what is wrong.

• Corrections ledgerRelease 6
• Corrections appliedRelease 6.1
• Reviewer confirmationsRelease 6.2
• Schedule 10 entries and reviewer notesRelease 6.3
• Records sent for reviewRelease 5.2
• Ground visits and contributionsRelease 5

Releases

ALTO publishes numbered data releases. Each release has a dataset ID, a publication date and a methods page that says what changed, the methods, the checks and the limits. The map and its pages carry their own tool version; the release metadata records both. Releases 5 to 6.3 redraw or relink crowns of the same 2024 survey, so a change in the number of crowns between releases does not establish tree loss, new planting or growth.

1. Release 6.3: Benefit value, height checks and group areas

   17 Oct 2026

2. Release 6.2: Reviewer-confirmed crowns and palm-type crowns

   13 Oct 2026 · Current release

3. Release 6.1: Permanent tree numbers

   13 Oct 2026

4. Release 6: Growth form and smoothed crown outlines

   12 Oct 2026

5. Release 5.2: Linking records to crowns

   10 Oct 2026

6. Release 5.1: Restoring links between records and crowns

   8 Oct 2026

7. Release 5: Redrawing Auckland’s canopy

   23 Sep 2026

• MethodsEvery release, newest first
• Release metadatarelease-metadata.json · current release