Calculating Area in the Browser — Shapefile & GeoJSON to m², ha, acres
"How many acres is this parcel?" sounds like a simple question, but measuring area accurately in GIS has more pitfalls than it looks. The exact same polygon can come out with an area that's off by several percent depending on the coordinate system it's measured in. This guide covers the underlying principle, and how to measure area directly in the browser without installing QGIS.
Why does area come out wrong?
The most common mistake is calculating area while the data is still in latitude/longitude (EPSG:4326). The unit of lat/lon is "degrees." One degree of latitude is about 111 km almost everywhere, but one degree of longitude covers a real-world distance that shrinks as you move away from the equator — about 111 km at the equator, but only around 88 km at Seoul's latitude. If you compute area as "degree × degree" in this state, the result doesn't match the true area on the ground.
That's why area must always be measured in a meter-based projected coordinate system. In Korea, the standard approach is to convert to Korea Central Belt TM (EPSG:5186) or UTM-K (EPSG:5179) first, then calculate. If coordinate systems are still a bit fuzzy, it's worth reading the coordinate systems guide first.
Area unit conversions
Calculations usually come out in square meters (m²), but in practice people also want hectares and acres side by side.
| Unit | Conversion |
|---|---|
| 1 hectare (ha) | 10,000 m² |
| 1 acre | ≈ 4,047 m² |
| 1 km² | 100 ha = 1,000,000 m² |
| 1 pyeong (Korean unit) | ≈ 3.3058 m² |
Step by step: measuring area in the browser
- Prepare the file: for a Shapefile, bundle
.shp·.shx·.dbf·.prjtogether into a single.zip. GeoJSON and KML just need the one file. - Upload: drag the file into the browser window. It's never sent to a server — everything is processed inside your browser, which matters if you're working with sensitive survey data.
- Automatic coordinate system detection: the CRS is inferred from the
.prjfile and the coordinate range. Area calculation itself is done internally in a meter-based coordinate system. - Read the result: per-polygon area is shown in m², hectares, and acres, along with a total, and can be exported as a table (CSV/Excel).
An honest note on accuracy
GISDirect's area calculations match standard GIS engines like GEOS/Shapely to within 0.2% when cross-checked. That said, this is not the same thing as a legally certified survey result — the precision of the source data itself (digitizing error, loss from coordinate transformations) sets the upper bound on how accurate the result can be. It's more than sufficient for planning, review, and rough estimates, but for official purposes like cadastral finalization, an actual survey result should be the authoritative source.