GIS glossary: 44 terms, plainly defined

Attribute table

The spreadsheet-like table of non-spatial data attached to a vector layer: one row per feature, one column per field. In a shapefile it lives in the .dbf sidecar; exporting it is the whole point of shapefile to CSV.

Bounding box (extent)

The smallest rectangle that contains every feature in a layer, given as minimum and maximum X and Y. GIS software uses it to zoom to a layer, and a wildly wrong extent usually means a coordinate-system mismatch.

.cpg file

A tiny shapefile sidecar that records which character encoding the attribute table uses (for example UTF-8). Without it, accented names and non-Latin text can turn to garbage when the .dbf is opened elsewhere.

Coordinate reference system (CRS)

The complete recipe that ties coordinates to real places: a datum plus, for projected systems, a map projection. When data lands in the wrong place, the CRS is almost always the culprit; see the guide to coordinate systems and EPSG codes.

Datum

The reference model of the Earth's shape that a CRS is anchored to, WGS84 and NAD83 are the ones you will meet most. Two datasets on different datums can disagree by anything from centimeters to a few hundred meters.

.dbf file

The dBASE-format table holding a shapefile's attributes. It caps field names at 10 characters and text fields at 254, which is why long column names get truncated when you convert GeoJSON to shapefile.

Decimal degrees (DD)

Latitude/longitude written as a single decimal number, such as 33.6844, 73.0479. It is the format GeoJSON, KML and most software expect; the coordinate converter translates between DD and the degree-minute forms.

Degrees minutes seconds (DMS)

The traditional coordinate notation, 33°41'04"N 73°02'52"E, still common on paper maps, deeds and survey reports. See latitude and longitude formats for how DD, DMS and DDM relate.

Degrees decimal minutes (DDM)

A halfway notation, 33°41.07'N, used by marine and aviation GPS receivers. Easy to confuse with DMS at a glance; the digits after the degree symbol mean different things in each.

DGN

MicroStation's CAD design format, organized by levels and elements rather than an attribute table. The converter reads DGN V7; save a V8 file as V7 or export it to DXF first. See DGN to Shapefile.

DXF

Autodesk's Drawing Exchange Format, the interchange format of the CAD world. It stores geometry in drawing units with no attribute table and usually no CRS, which drives most of the checks on DXF to shapefile. Background: what is a DXF file?

EPSG code

A short number that unambiguously names a CRS, 4326 is WGS84 latitude/longitude, 3857 is web-map Mercator, 32643 is UTM zone 43N. Far safer to quote than a CRS name, and searchable at epsg.io.

Feature

One thing on the map: a single point, line or polygon together with its attributes. A layer is a collection of features of the same geometry type.

FeatureCollection

The top-level object of a GeoJSON file, an array of features wrapped with a type declaration. It is what you get from any conversion to GeoJSON on this site, such as KML to GeoJSON.

GDAL / OGR

The open-source translator library that virtually every GIS package uses under the hood for format reading and writing. GISConverters runs a full GDAL build compiled to WebAssembly inside your browser; see how the site works.

Geocoding

Turning an address or place name into coordinates (and reverse geocoding is the opposite). Distinct from format conversion: this site converts files that already contain coordinates, it does not look addresses up.

GeoJSON

A JSON-based vector format, defined by RFC 7946, that web-mapping libraries read natively. Human-readable, one file, always WGS84; the full story is in GeoJSON format explained.

Geometry type

Whether features are points, lines or polygons (or their multi-part variants). Formats differ in tolerance: GeoJSON mixes types freely in one file, while a shapefile stores exactly one type per layer, a key check when you convert KML to shapefile.

GeoPackage (.gpkg)

A modern OGC format that packs any number of layers, of any size, into a single SQLite database file. Increasingly the default advice instead of shapefiles; compared head-to-head in GeoPackage vs shapefile.

GPX

The GPS Exchange format, an XML schema for waypoints, routes and tracks that practically every GPS device and fitness app speaks. It stores WGS84 coordinates only and has no polygon type; see the GPX file explained.

KML

Keyhole Markup Language, the plain-text XML format Google Earth and Google Maps read, always in WGS84. The site's most-converted input; every pairing lives on the KML & KMZ converter.

KMZ

A KML zipped into one archive, often with its icons and overlays bundled beside the document. Smaller to email and the default Google Earth export; see KML vs KMZ.

Latitude & longitude

The angular coordinates of a point on the Earth: latitude north/south of the equator, longitude east/west of Greenwich. The perennial trap is axis order, GIS tradition says lat/lon, but GeoJSON and most file formats store lon/lat (X first).

Layer

A set of features of one geometry type sharing one attribute schema and one CRS, the unit that GIS software adds to a map. Some formats hold one layer (shapefile), others hold many (GeoPackage, KML, DXF).

Multi-part geometry

A single feature made of several shapes, a MultiLineString for a route with gaps, a MultiPolygon for an island group. Converters often promote lines to multi-lines for consistency, which is normal and lossless.

Null Island

The nickname for 0, 0, in the Atlantic off West Africa. Points land there when coordinate fields are empty or unread, usually after a spreadsheet import; the wrong-place guide has the fix.

Placemark

KML's word for a feature: a named point, path or polygon with a description balloon and a style. Spreadsheet rows become placemarks when you convert CSV to KML.

.prj file

The shapefile sidecar holding the layer's CRS as WKT text. Losing it is the single most common cause of data landing in the wrong place; the rescue procedure is in GIS data in the wrong place.

Projection (projected CRS)

A mathematical flattening of the curved Earth onto a plane so positions can be stated in meters or feet, UTM zones and national grids are examples. Projected coordinates look like large numbers (500000, 3730000) rather than degrees.

Raster vs vector

Raster data is a grid of pixels (satellite imagery, elevation models); vector data is geometry (points, lines, polygons). This site's converters are vector-focused, since raster imagery needs different handling.

Reprojection

Recomputing coordinates from one CRS to another, degrees to UTM meters, for instance. It is a mathematical transformation, not a relabelling; assigning the wrong CRS name without transforming is how data ends up in the sea.

Route (GPX)

A planned path made of a few chosen points, which a navigation device follows turn by turn. The recorded counterpart is a track; see the GPX file explained.

Shapefile

Esri's 1990s vector format, still everywhere, that is really a family of files (.shp, .shx, .dbf, .prj, .cpg) which must travel together. Its rules and quirks are covered in what is a shapefile?

.shx file

The shapefile's spatial index, a lookup table that lets software jump straight to feature number N in the .shp. It is required: many programs refuse to open a shapefile without it.

SRID

The spatial reference identifier a database uses to tag geometry with its CRS, in practice usually the same number as the EPSG code (4326 and friends). You will meet it in PostGIS and GeoPackage metadata.

Track (GPX)

A recorded path in a GPX file: an ordered list of trackpoints with optional timestamps and elevations, i.e. where you actually went. Routes are the planned counterpart; the difference matters when you convert GPX to shapefile.

UTF-8 encoding

The character encoding that can represent every script, and the safe choice for attribute text. CSV exports here are UTF-8, and shapefile output includes a .cpg declaring it, so names like "Kärnten" or "كراچی" survive conversion.

UTM

The Universal Transverse Mercator system: 60 numbered zones, each a projected CRS in meters. Survey and drone data very often arrives in a UTM zone, which is why spreadsheet converters here ask you to confirm the CRS when coordinates look projected.

Vertex

A single coordinate pair along a line or polygon boundary; geometry is stored as ordered vertex lists. Simplification removes vertices to shrink files, at the cost of shape fidelity.

Waypoint

A single named point in GPS data, a summit, a sample site, a cache. Waypoint lists move between spreadsheets and devices via CSV to GPX and back.

Web Mercator

The projected CRS web maps draw in, EPSG:3857, in meters. Good for display, wrong for measuring: a Web Mercator meter equals a ground meter only at the equator. Convert with lat/long to Web Mercator.

WebAssembly (WASM)

A browser technology that runs compiled native code at near-native speed. It is what makes this site possible: the same GDAL engine a desktop GIS uses, executing inside your tab so files never need uploading.

WGS84

The World Geodetic System 1984, the datum and lat/lon CRS used by GPS and assumed by KML, GPX and GeoJSON (EPSG:4326). When in doubt about a web or GPS dataset's CRS, WGS84 is the usual answer.

WKT (well-known text)

A text notation for geometry, POINT(73.05 33.68), LINESTRING(…), that fits in a spreadsheet cell. This site both reads WKT columns from CSV/Excel input and writes them (as the last column) when exporting shapes to tables. The same notation, confusingly, is also used to describe a CRS inside a .prj file.