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Buffer Tool

The Buffer tool generates a polygon buffer at a chosen radius around an existing feature - point, polyline, or polygon. The result is a new polygon inserted into your active polygon layer. Common uses are proximity zones, stand-off boundaries, no-build zones, vegetation buffers, and catchment polygons around point features.

Pro+ feature

Buffer is part of the Advanced Editing Pro+ feature set. Free and Pro users can see the master toggle in Settings > Advanced Editing but turning it on opens the subscription paywall.

When to use Buffer​

  • A proximity zone around a point of interest (e.g. 50 m around a monitoring well, 100 m around a tree).
  • A stand-off zone along a line (e.g. road kerb buffer, pipeline exclusion zone).
  • A growth area around an existing polygon (extending a parcel by 10 m on every side).
  • A shrunk area inside an existing polygon (a stand-off zone within a parcel - requires the Allow negative radius setting).
  • A ring / donut around an existing polygon (the area within N metres of the boundary, but not the polygon's interior).

Before you start​

To use Buffer you need:

  • A Mapit Pro Plus subscription.
  • The Advanced Editing master toggle on in Settings > Advanced Editing.
  • A polygon layer set as the active layer. The buffer is always a polygon and is always written into the active layer; if no polygon layer is active, the Buffer entry shows in the editing-tools sheet with a hint reading "Set a polygon layer as active to use Buffer".

The source feature can be on any layer - the source layer and the destination polygon layer do not need to match. A typical workflow is "tap a point on the points layer, set a polygon layer as active, buffer the point into a 50 m polygon zone in the polygon layer".

The flow​

The entry point depends on the source geometry type.

Polygon or polyline source​

  1. Tap an existing polygon or polyline feature to start an edit session on it.
  2. The right-side toolbar shows the usual edit FABs plus a Tools chip near the top.
  3. Tap Tools to open the editing-tools bottom sheet. With v1 the sheet shows two entries: Split and Buffer. The Buffer entry is enabled when your active layer is a polygon.
  4. Tap Buffer. The regular FABs are hidden and a compact card slides up above the bottom navigation: title "Buffer feature", the current radius value (default 50 m), a slider, and Cancel / Apply buttons.

Point source​

  1. Tap an existing point feature. The standard Edit Point Coordinates dialog opens (the same dialog Mapit uses to edit a point's lng/lat directly).
  2. Below the coordinate inputs, when the active layer is a polygon and you have Pro+, you will see an Advanced editing tools section with a small-print line "Available: Buffer" and an outlined Open advanced tools button.
  3. Tap Open advanced tools. The dialog closes and the editing-tools sheet opens in point-source mode - only the Buffer entry is shown (Split is meaningless for points).
  4. Tap Buffer to open the same Buffer card.

Adjust the radius​

Drag the slider. The buffered polygon is recomputed and re-rendered on every tick - usually sub-millisecond per recompute. The preview is drawn in teal with a slightly darker outline so it reads clearly against your basemap and does not clash with the green / blue colours the Split preview uses.

The slider range is 1 to 500 m by default. If you turn on Allow negative radius in the settings AND the source is a polygon AND the shape is Filled, the slider expands to -500 to +500 m, where negative values shrink the polygon inwards.

The Apply button is enabled only when the current radius produces a valid (non-empty) buffer.

Apply or Cancel​

  • Apply writes a single new row to the active polygon layer with the current buffer geometry and immediately re-renders the layer so the new feature appears on the map. A toast confirms with the destination table name. Attributes for the new row are blank by default - the buffer is a fresh feature, not a copy of the source.
  • Cancel discards the preview without writing. The polygon / polyline edit session is restored (you can keep editing the source); the point-source flow returns you to the bare map.

Buffer shape: Filled vs Ring​

The Buffer shape setting controls what the algorithm produces:

  • Filled (default) - the standard fat buffer polygon. Source plus a margin of the chosen radius.
  • Ring - the donut shape around a polygon source: the buffer polygon minus the original polygon. The result is just the band of area within N metres of the boundary, with the source's interior left out. Useful for vegetation buffers and "everything within N metres of the parcel edge" workflows.

Ring shape only makes sense for polygon sources with a positive radius. The slider gating in the toolbar enforces both: ring sources never see negative slider values.

For point sources, shape is forced to Filled regardless of the setting (a ring around a single point is geometrically degenerate without an inner radius too).

Corner style​

The Corner style setting maps directly to JTS join-style:

  • Round (default) - smooth curved corners. Standard buffer look.
  • Mitre - sharp corners that extend the source's edges to a point. CAD-feel for cadastral / regular shapes (buildings, parcels look "right" with mitred buffers).
  • Bevel - clipped sharp corner. The corner is replaced with a flat edge between the offset edges.

Line end caps​

Only relevant when buffering polylines:

  • Round (default) - rounded semi-circular caps at each line end.
  • Flat - the buffer is clipped flat at the line endpoint. Useful for road centerlines buffered into a carriageway, where rounded ends look weird.
  • Square - the buffer extends past the line end by the radius distance, with a square cap.

Allow negative radius (shrink)​

Off by default. When on, AND the source is a polygon, AND the shape is Filled, the slider exposes negative values. A negative radius shrinks the polygon inwards by that distance. Useful for stand-off zones inside a parcel ("everything more than 5 m from the boundary").

The slider gating is conservative on purpose: shrink is only valid for filled-polygon sources, so the toolbar shows the full -500 to +500 range only in that combination. Points, lines, and ring-shape sources always see the positive-only 1 to 500 range.

Accuracy notes​

Mapbox features carry geometry in WGS84 lng/lat. A radius value in metres needs to be converted to degrees before JTS can apply it. Mapit does this with a closed-form metric pre-scaling at the source's latitude:

  • The source coordinates are scaled into a metric space so 1 unit = 1 metre on both axes at the source latitude.
  • JTS buffers in metres directly (truly isotropic in metres, regardless of latitude).
  • The result is unscaled back to lng/lat.

The result reads as a real circle on the map at any latitude. This matters at non-equatorial latitudes (UK, Scandinavia, Canada), where 1 degree of longitude is significantly shorter than 1 degree of latitude - without this pre-scaling, a 370 m buffer at lat 58°N would appear as a noticeable N-S elongated ellipse.

The accuracy is sub-1% for typical urban-scale buffers (radius < 1 km) at temperate latitudes - well within the precision range field surveying actually needs.

Limitations in v1​

  • Multipolygon / multilinestring features are not supported as sources - the Tools chip / dialog section is hidden for them.
  • WFS layers as sources never see Buffer - they are read-only.
  • Multi-ring buffers (10 m, 20 m, 30 m at once) are not supported.
  • Buffer + dissolve across multiple sources is not supported.
  • Single-sided line buffer (one side of a line only) is not supported - both sides are always buffered.
  • Smoothness of round corners is fixed at JTS' default (8 segments per quadrant, ~32-sided circle approximation).

Requirements​

  • A Mapit Pro Plus subscription (Android).
  • Advanced Editing master toggle on in Settings > Advanced Editing.
  • A polygon layer set as active (the destination for the buffer).