Appendix 1 2(a), 2(b) - Geospatial data, maps and GIS

Location and attributes

Data become geographical when they describe a located phenomenon, its spatial relationships or its changes through space and time. A temperature of 18°C needs a location, observation time and measurement conditions before it can support a comparison between an urban centre and its surroundings.

An attribute describes a feature; geometry describes its spatial form. A sampling station can be a point with attributes for time and temperature, a river a line with flow attributes, and a ward a polygon with population attributes. A location alone does not explain a process: nearby places may share geology, but they may also have different drainage or land management.

Latitude records angular position north or south of the equator; longitude records angular position east or west of the prime meridian. Record the coordinate order explicitly. In a grid, eastings increase eastwards and northings increase northwards. A coordinate reference system (CRS) specifies how coordinates relate to Earth. A geographic CRS uses angular coordinates; a projected CRS represents positions on a plane, commonly in metres. A degree of longitude does not have the same ground distance at every latitude. Choose a suitable projected CRS for a local distance or area calculation and check that imported layers align.

A four-figure grid reference identifies a square: read the easting first, then the northing. For a six-figure reference, divide each kilometre square into tenths. A point 300 m east and 700 m north of the southwest corner of square 24/61 has reference 243617. The reference identifies position to roughly 100 m, not to the nearest metre.

Scale, direction and relief

A map reduces space to a scale, so measurements require conversion before they describe the ground. A scale of 1:25,000 means 1 cm on the map represents 25,000 cm, or 250 m, on the ground. Thus a 6.4 cm straight line represents 1,600 m or 1.6 km. Measure a winding route with short straight segments or a suitable tracing tool; a straight line between endpoints underestimates its length. A scale bar remains useful after proportional resizing, whereas a printed representative fraction no longer applies if the page size changes.

Bearings are measured clockwise from north: east is 090°, south 180° and west 270°. Identify whether the map uses true, grid or magnetic north; these are not automatically identical. For a northeast line making an angle of 35° clockwise from grid north, the grid bearing is 035°.

Contours connect points of equal elevation. Close contours usually indicate a steeper gradient than widely spaced contours when the contour interval and map scale are the same. Read labelled heights before inferring a hill or valley; absence of contours on a simplified map does not prove level ground. Contour patterns, rivers and settlement positions together can support an explanation of access or exposure.

Map interpretation should separate observations from explanations. “The buildings lie beside the river” is a mapped observation; “they were built there for transport access” needs historical or land-use evidence.

GIS models and layers

A geographical information system (GIS) stores, combines, analyses and presents located information. Its value comes from asking a spatial question across compatible layers, rather than simply displaying a colourful map.

Vector data use points, lines and polygons. They suit features with recognisable boundaries, such as sites, roads and administrative areas. Raster data divide space into cells, each holding a value: elevation, reflectance, rainfall or a land-cover class. Raster cell size affects detail. A 100 m cell covering several land uses cannot identify every small garden within it. Vector boundaries also have uncertainty; drawing a precise line does not guarantee a precise survey.

Start an investigation by defining the target, such as settlements near a river and on low ground. Obtain dated layers, inspect units and CRS, correct missing or duplicate records, and join attribute tables using a stable identifier. A buffer identifies space within a stated distance of a feature. An overlay combines layers; an intersection retains their shared area. A spatial join can attach the polygon containing each site to its point record. Attribute queries then select records meeting conditions. Preserve the original data and record each analytical choice so someone else can reproduce the result.

Distance must match the question. A 500 m circular buffer around a clinic measures straight-line proximity. Walking access may depend on bridges, road connections and barriers; a network analysis is more appropriate. Likewise, a point inside a hazard outline is exposed according to that model, not certain to suffer damage.

A worked spatial selection

Combining distance and elevation gives a more specific selection than using either layer alone. The following local grid and household totals are original illustrative data, not an actual flood survey. Coordinates are in kilometres in a suitable local projected grid.