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.

Android

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).

iOS

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, then tap Edit geometry from the feature info menu to start an edit session.
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 three entries: Split, Buffer, and Merge. The Buffer entry is enabled when your active layer is a polygon.
4. Tap Buffer. The regular edit FABs are replaced by a compact card above the bottom safe area with a prompt, the current radius value (default 10 m), a slider, and Cancel / Apply buttons.

Point source

iOS does not have a textfield-based coordinate-edit dialog like Android, so the point-source entry is in the feature info sheet instead of in a dialog:

1. Tap an existing point feature. The feature info sheet slides up with a row for the tapped point.
2. Tap the ellipsis menu on that row. When the active layer is a polygon and you have Pro+, a Buffer action appears in the menu alongside Edit geometry, Edit attributes, etc.
3. Tap Buffer. The feature info sheet dismisses and the Buffer card opens directly — no edit session needs to be started.

The Buffer flow from there is identical to the polygon / polyline entry point.

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 blue with a matching 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 and can be expanded to 100 m – 10 km via the Maximum radius setting. If you turn on Allow negative radius AND the source is a polygon AND the shape is Filled, the slider expands to symmetric -max … +max, 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. Attributes for the new row are blank by default — the buffer is a fresh feature, not a copy of the source. For the polygon / polyline flow the edit session is also committed at this point (any in-flight edits to the source land too); for the point-source flow there was no edit session to begin with, so nothing else changes.
• Cancel discards the preview without writing. For the polygon / polyline flow the 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 GEOS 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 symmetric negative range only in that combination. Points, lines, and ring-shape sources always see the positive-only range.

Accuracy notes

Mapbox features carry geometry in WGS84 lng/lat. A radius value in metres needs to be converted to degrees before GEOS 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.
• GEOS 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 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 the GEOS default (8 segments per quadrant, ~32-sided circle approximation).

Requirements

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

---

Related Topics

• Split Operation - Cut an existing polygon or polyline into two features.
• Merge Tool - Combine two adjacent features on the same layer into one.
• Advanced Editing Settings - Configure buffer shape, corner style, line end caps, and the allow-shrink toggle.
• Vertex Snapping - Snap new vertices to existing features for clean topology.
• Mapit Pro Plus - What is included in the Pro+ subscription.