Downloading a site boundary as GeoJSON for an application

A machine readable site boundary is the least glamorous useful thing you can produce at the start of a project. One polygon, exported as GeoJSON, gives every consultant on the job the same red line, lets you check the site area against the figure you are about to write on the application form, and provides the geometry for constraint queries against the national planning datasets.

It takes about ten minutes. The reason to spend them is that a site area stated wrongly on a form is a validation query, and a red line drawn slightly differently by five consultants produces five slightly different constraint searches.

Why a digital boundary saves time later

Four things follow from having the polygon.

• Every consultant works from the same geometry, so the ecologist's survey area, the arboriculturist's plan and the drainage catchment all start from the same line.
• You can query the national planning datasets directly for designations intersecting the boundary, rather than reading them off a map by eye.
• The area can be calculated rather than estimated, which is what the application form asks for.
• The same polygon can be restyled for the location plan and the block plan without redrawing.

The counterpoint is worth stating. A digital boundary is a planning tool, and it is not evidence of ownership. That remains a matter for the title plan, and the two need reconciling rather than assuming they agree.

Sources of parcel and building footprint data

Three kinds of open data are useful, and each has limits.

Parcel and land data published through the national planning data platform covers designations, allocations and some boundaries, contributed by individual authorities. Coverage is uneven, so treat a nil result as an absence of published data rather than an absence of the thing itself.

Building footprints and street data from open mapping sources are accurate enough to trace a building outline and to sanity check a survey, and they are generally not accurate enough to set a boundary on. Where a boundary follows a physical feature such as a wall or a hedge centre line, a topographical survey is the only reliable source.

Satellite and aerial imagery is useful for tracing a field boundary or a yard edge, provided you check the date of the imagery and account for the lean of tall buildings away from the camera. A roof edge is not a wall line.

Constraint layers such as flood risk and heritage designations are best queried against your finished polygon rather than used to draw it. Our note on checking whether a watercourse is a main river shows how that kind of query works in practice for one dataset.

Drawing and exporting a polygon as GeoJSON

Any mapping tool that exports GeoJSON will do, whether that is a desktop geographic information system, a browser based editor or a survey package. The output is a plain text file describing a single polygon with a list of coordinates.

A workable sequence.

1. Load a base map and, if you have one, the topographical survey as a reference layer.
2. Trace the boundary as a single closed polygon, following the features the boundary actually follows.
3. Include any land you need for access, visibility splays or drainage outfall, even where it is in different ownership, and note it separately.
4. Snap vertices to the survey rather than to the imagery wherever the survey covers them.
5. Export as GeoJSON, then open the file in a text editor to confirm it contains one feature rather than several.
6. Name the file with the site, the date and a version number.

Keep the number of vertices sensible. A boundary traced with three hundred points around a straight garden wall is harder to work with and no more accurate than one with four.

Coordinate systems and why they matter

This is where exports go wrong. GeoJSON is defined to use longitude and latitude on the WGS 84 datum, with longitude first. British survey data and Ordnance Survey mapping use eastings and northings on the British National Grid.

Two consequences.

If you export British National Grid coordinates into a GeoJSON file without reprojecting, anything that reads the file according to the specification will place your site somewhere in the Atlantic or fail outright. And if you export correctly but then calculate area from the longitude and latitude values directly, the figure will be wrong, because degrees are not metres and the scale factor changes with latitude.

So do two things. Reproject to WGS 84 for the GeoJSON file itself, and calculate areas in a projected system such as the British National Grid, where a metre is a metre. Record which system each figure came from.

Where coordinates appear in the reverse order to what you expect, a coordinate pair reversal is almost always the cause, and it is visible immediately because the site appears in the wrong hemisphere.

Checking area against the figure on your form

The application form asks for the site area, and it is worth calculating rather than repeating whatever appeared on an earlier drawing.

Calculate the area from the polygon in a projected coordinate system, in hectares to three decimal places, then compare it with three other figures: the area shown on the title plan, any area quoted in the agent's earlier correspondence, and the area implied by the scale bar on the submitted location plan.

Where the figures disagree, find out why before submitting. The usual causes are a boundary that includes or excludes an access strip, a red line drawn to the centre of the highway rather than to the back of footway, and a title that includes land the application does not.

Two thresholds make the area figure matter more than it looks. A site of 0.5 hectares or more is capable of being major development for procedural purposes, and a site of one hectare or more triggers a flood risk assessment requirement irrespective of flood zone. A site area recorded as 0.49 hectares when it is actually 0.52 is a problem that surfaces at exactly the wrong moment.

Reusing the boundary across consultant packages

Send the polygon out with the brief, not after the first draft.

Include it in the information pack alongside the survey, the title plan and the draft layout, with a short note stating the coordinate system, the date, the version and what the boundary includes. Ask each consultant to confirm they are using that version, and reissue with a new version number if the boundary changes.

Then use it yourself for the constraint check. Querying the planning datasets against the polygon gives you a designation list you can hand to every author, and it removes the situation where the ecologist and the heritage consultant have each drawn their own search radius from a different centre point. Our notes on historic maps as former land use evidence and on checking strategic noise maps before layout both assume a fixed boundary to work from.

Limits of open data for legal ownership

Be clear with yourself about what the polygon is not.

It is not the title boundary. Registered title plans show general boundaries rather than precise lines, and a general boundary and a surveyed feature can differ by a metre or more. It is not evidence of ownership or of rights of access. It does not tell you about covenants, easements or unregistered interests. And it does not answer the ownership certificate question on the application form, which turns on legal ownership at a date, not on where a line sits on a map.

So the polygon supports the planning work and the title work runs alongside it. Where land outside your ownership is included in the red line, serve the required notice on the other owner and complete the correct certificate. Where rights of way cross the site, the definitive map is the record, and our note on definitive map data for public rights of way covers how to check it.

One honest caveat about open data generally. Coverage and currency vary by authority and by dataset, so a search that returns nothing has told you something about the data rather than something about the site.

This article is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.