Aerial Photographs

Types of Aerial Photography and Satellite Imagery

Aerial photographs appear on the examination in three distinct formats, each providing a different vantage point.

Vertical Aerial Photographs

Captured from directly overhead with the camera facing straight down, vertical photographs offer a planimetric view that closely resembles a map. Distances and directions are reliable in the centre of the image, although some distortion occurs towards the edges. Because scale remains relatively consistent across flat terrain, vertical images are well suited to measuring field boundaries, surveying land use, and calculating surface areas.

Oblique Aerial Photographs

Taken at an angle to the ground, oblique photographs provide a natural, three-dimensional perspective that reveals the height, architectural shape, and relief of features.

• Low oblique: The camera tilts down at a steep angle so that the horizon is excluded from the frame.
• High oblique: The camera captures a broader view that includes the horizon line near the top of the frame.

Satellite Imagery in Weather and Climate

Recorded from orbital sensors, satellite images monitor large-scale physical systems across wide regions. In weather forecasting, meteorologists rely on three main forms:

• Visible imagery: Measures reflected sunlight. Thick cloud layers appear bright white, while land and sea appear dark. This feed is available only during daylight hours.
• Infrared imagery: Measures radiant temperature. High, cold cloud tops appear brilliant white, whereas warm ocean and land surfaces appear dark. This imagery is operational day and night.
• Water vapour imagery: Tracks atmospheric moisture in the middle troposphere, making it ideal for plotting the path of the jet stream.

On synoptic weather imagery, a mid-latitude depression appears as a distinctive comma-shaped cloud pattern spiralling anticlockwise around a centre of low pressure. Its warm front forms a wide band of layered stratiform cloud yielding steady rain, while its trailing cold front appears as a narrow, intense line of towering cumulonimbus clouds delivering heavy downpours. A cloud-free, clear sector indicates an anticyclone.

Locating Features and Cross-Referencing with OS Maps

You cannot give a grid reference for a photograph on its own. Instead, you locate features on the photograph using the nine-section system, identify those same features on the accompanying 1:50 000 OS map, and take your measurements and grid references directly from the map.

The Nine-Section System on Oblique Images

Perspective distorts scale and orientation across an oblique image, so standard compass directions cannot be used. We divide the image into nine zones by pairing horizontal positions (left, centre, right) with depth bands:

Vertical Photograph Orientation

On a vertical photograph the camera points straight down, making the image map-like. Before applying compass sectors (North-West, North, North-East, West, Centre, East, South-West, South, South-East), check the orientation of the photograph by matching a distinctive line feature, such as a river, railway, or main road, with the same feature on the OS map. If the top of the photograph does not point north, rotate your compass sectors to match the map.

Finding Camera Direction

Work out which direction an oblique camera was pointing using five steps:

1. Pick a clear landmark in the foreground of the photograph, such as a church or bridge.
2. Pick a second prominent feature in the background along the direct line of sight.
3. Find both matching features on the accompanying 1:50 000 OS map.
4. Draw a straight line on the map running from the foreground feature to the background feature.
5. Read the direction of that line using the OS map's north arrow. For instance, if the line runs from a church toward a lake situated to the north-west, your camera was pointing north-west.

OS Map Grid References and Measurements

Examiners routinely set 2-mark questions requiring map coordinates and measurements for landmarks seen on the photograph:

• Rule for grid references: Read eastings first, then northings — remember "along the corridor and up the stairs". Vertical grid lines are eastings, numbered left to right. Horizontal grid lines are northings, numbered from bottom to top. Always start with the correct sub-zone letter (such as H, M, S, W, or V) printed on the sheet; omitting the letter forfeits the marks.
• Four-figure grid reference: Identifies a whole 1 km × 1 km grid square. Take the easting line immediately to the left of your feature and the northing line immediately below it (for example, H 42 04).
• Six-figure grid reference: Pins down an exact location to the nearest 100 metres. Mentally divide the grid square into tenths. Read the easting followed by the tenths across, then the northing followed by the tenths up (for example, M 122 427).
• Road numbering: Look closely at the route classifications on the map. National primary roads carry an N prefix with thick casing (such as the N71), national secondary roads show an N prefix with thinner casing, regional roads carry an R prefix (such as the R198), and motorways use an M prefix.
• Distance: On a 1:50 000 map, 2 cm equals 1 km. For straight-line distances, measure with a ruler in centimetres and divide by 2. For winding roads or rivers, run the edge of a strip of paper along the bends, mark the start and end points, measure the total paper length, and divide by 2.
• Area: Each full grid square equals 1 km². Count the complete squares, count the partial squares and divide them by 2, then add both numbers together to give total square kilometres.
• Aspect: This represents the compass direction a hillside faces. Follow the decreasing contour values downhill to determine which way the slope falls.

Constructing Sketch Maps and Reading Profiles

A sketch map tests whether you can extract the primary spatial features from a source and record them accurately in proportion. Show only what the question specifies.

Drawing a Sketch from an Aerial Photograph

1. Measure and scale: Measure the length and width of the printed photograph with your ruler. Halve both values to establish your frame dimensions. If the photograph is 20 cm by 14 cm, your sketch frame must be exactly 10 cm by 7 cm.
2. Pencil grid: Lightly draw a 3×3 grid of nine equal rectangles across your sketch frame to help place features in their correct relative positions.
3. Coastlines and rivers: Draw major water boundaries first to anchor the landscape.
4. Linear features: Draw roads and rivers as single, clean pencil lines. Drawing them as thick ribbons distorts the sketch and loses presentation marks.
5. Labels and legend: Outline the requested features, shade them lightly, and construct a complete legend underneath.
6. Direction indicator: Do not add a north arrow to a sketch of an oblique aerial photograph unless the camera direction is explicitly stated or printed on the original image.

Drawing a Sketch from an OS Map

When drawing a sketch from an OS map extract, draw the frame to the proportions instructed (usually half-scale). Unlike an oblique photograph sketch, a sketch drawn from an OS map must always include a north arrow, because grid north is always at the top of an Ordnance Survey sheet. Give the sketch a clear title, plot only the features requested, and label them clearly.

Elevation Profiles and Slope Types

Cross-sections or elevation profiles require you to identify slope shapes from contour spacing:

• Concave slope: Contours are tightly packed at the top of the hill and widely spaced near the bottom, creating a steep upper gradient that flattens out towards the base.
• Convex slope: Contours are widely spaced at the summit and closely packed near the base, producing a gentle upper crest that drops off steeply below.
• Even/uniform slope: Contours are evenly spaced from top to bottom, indicating a steady, constant angle of incline.
• Stepped slope: Alternating bands of closely packed and widely spaced contours indicate successive rock ledges, terraces, or plateau edges.

Settlement Patterns, Functions, and Site Selection

Settlement geography questions ask you to explain why a town developed where it did, identify its ongoing economic functions, and select suitable sites for future expansion.

Site and Situation

• Site: The actual physical ground on which the settlement is built. Common site advantages visible on photographs and maps include a river bridging point where streets converge, a dry-point knoll situated above the damp floodplain, flat lowlands that reduce construction costs, natural defensive positions inside river meander loops, or sheltered coastal inlets suitable for quays.
• Situation: The position of a settlement relative to surrounding regions and transport links. A classic situation advantage is a nodal route focus where several regional roads and a national primary road converge on a bridging point, allowing the town to function as a central market place serving an extensive agricultural hinterland.

Rural Settlement Patterns

Four rural settlement patterns recur on Irish photographs and maps:

• Nucleated: Buildings cluster closely around a central focal point, such as a road junction, bridge, church, or market square.
• Linear/ribbon development: Houses and commercial premises form a continuous line along a single transport corridor leading out of a town.
• Dispersed: Individual farmhouses and cottages are scattered across open countryside, separated by fields and hedgerows, typical of rural western Ireland.
• Clustered/clachan: A tight cluster of farm dwellings and outbuildings grouped together without a formal street layout, church, or shop.

Photographic Evidence Bank for Settlement Functions

In the exam, state the function, identify its photograph sector, and supply visual evidence:

Suggesting a Suitable Development Site

When recommending a site on a photograph for a new supermarket, hospital, or industrial park, state the exact sector and justify the choice under three headings:

1. Space: Select a flat greenfield site on the outskirts of town that provides ample room for single-storey construction, customer car parking, delivery access, and future expansion.
2. Access: Confirm the site borders an N road, R road, or ring-road junction, allowing easy freight delivery and customer access while keeping heavy commercial traffic away from narrow centre streets.
3. Services and amenity: Check that water, drainage, and electricity connections can easily link into nearby networks. Point out that placing commercial units on the periphery prevents disturbance to quiet residential streets.
4. Counter-argument: State clearly that the chosen site avoids building on steep slopes and eliminates the flood risk visible along the low-lying river floodplain.

Recognising Physical Landforms and Seasonal Clues

Because photographs do not carry printed symbols, you must identify geomorphological landforms and seasons from shapes, shadows, and textures.

Coastal Landforms

A vertical rock face fronted by breaking waves marks a sea cliff, often accompanied by a flat, wave-cut platform exposed at low tide. Wave refraction concentrates energy onto headlands, eroding sea caves, natural arches, and isolated sea stacks. Along depositional coastlines, constructive waves and longshore drift form expansive sandy beaches backed by sand dunes covered in marram grass. A sand spit forms a narrow ridge extending into a bay with a curved, hooked end, occasionally trapping a shallow coastal lagoon. A sand deposit connecting an offshore island to the mainland forms a tombolo. Wooden or rock barriers erected perpendicular to the shoreline indicate groynes installed to prevent sediment transport.

Glacial, Fluvial, and Karst Landforms

• Glacial: Armchair-shaped mountain hollows represent corries, frequently holding a tarn lake. Knife-edged ridges dividing adjacent corries are arêtes, while wide, flat-floored valleys form U-shaped valleys. In lowland regions, swarms of smooth, egg-shaped hills form a drumlin landscape, while winding ridges of fluvio-glacial sand and gravel mark eskers. Bare, scoured bedrock often exhibits parallel striations.
• Fluvial: Meanders loop across broad, flat floodplains, occasionally leaving behind curved oxbow lakes. Natural embankments lining river banks represent levees. Where two watercourses meet is a river confluence, while a river splitting into distributaries at the sea creates a delta.
• Karst: Exposed limestone pavement appears as bare grey rock divided into rectangular blocks (clints) separated by deep fissures (grikes). Funnel-shaped depressions where surface streams sink underground mark swallow holes, while loose, angular rock fragments piling up at the base of bare rock faces form scree slopes.

Reading Seasonal Landscape Clues

• Spring: Arable fields reveal dark, freshly ploughed soil. Pastures contain newborn livestock, and hedgerows display fresh, pale green growth.
• Summer: Deciduous trees stand in full canopy. Grass has been cut for silage or hay, leaving pale, stripey mown rows, and round bales wrapped in black or white plastic lie in the fields. Bright yellow fields in early summer are oilseed rape in flower. The high midday sun casts short shadows.
• Autumn: Deciduous woodland displays rich golden, amber, and brown foliage. Cereal fields retain pale stubble following harvest, and early morning chimney smoke begins to appear.
• Winter: Deciduous branches are completely bare of leaves. The low angle of the winter sun causes trees, spires, and buildings to cast long, prominent shadows across the ground. Plumes of domestic chimney smoke drift steadily from residential roofs.

Comparing OS Maps and Aerial Photographs

Evaluating both sources allows geographers to monitor urban expansion, changing land uses, and infrastructure developments over time.

Contrasting Sources

Evaluating Advantages and Limitations

The primary advantages of an aerial photograph include showing the landscape as it truly looks in real time, displaying building heights on oblique shots, and revealing fine details omitted from maps, such as traffic congestion and crop ripening. Its key limitations are the absence of place names, contours, national grid lines, and legends, alongside scale distortion on oblique views and the possibility of shadows obscuring lower-lying features.

Conversely, an OS map provides reliable scale, precise grid references, and clear contours, but quickly becomes outdated as towns expand.

Evidence of Urban Sprawl and Landscape Change

When comparing an aerial photograph to an older OS map extract, search for clear evidence of modern growth:

• Modern housing estates and retail parks visible on the photograph occupy areas marked as open farmland on the map, proving the map was surveyed earlier.
• Unplanned ribbon development lines the regional approach roads leading out of town.
• A modern bypass or motorway routes heavy freight traffic around town, even though the older map shows all traffic funnelling through the core.
• Outer farmland is converted into greenfield site construction, whereas derelict town yards are cleared as brownfield site regeneration projects.
• Well-known Irish examples include rapid suburban sprawl across Adamstown and Blanchardstown west of Dublin, extensive commercial brownfield regeneration at Grand Canal Dock in the Dublin Docklands, pedestrianisation along Grafton Street, and modern congestion bypasses such as the M11 Enniscorthy bypass.

Key terms

Planimetric

A two-dimensional, map-like perspective where horizontal distances and positions across the terrain are geometrically uniform, characteristic of vertical aerial photographs and Ordnance Survey maps.

Vertical Aerial Photograph

An aerial photograph captured with the camera pointing straight down towards the ground, providing an even overhead view suitable for measuring surface areas.

Oblique Aerial Photograph

An aerial photograph taken at an angle to the landscape, offering a three-dimensional view that reveals building heights, facades, and terrain relief.

Low Oblique

An angled aerial photograph taken with a steep downward camera tilt that excludes the horizon line from the frame.

High Oblique

An angled aerial photograph captured with a broad view that includes the horizon line near the top of the frame.

Nine-Section System

A standardised geographical framework dividing an aerial photograph into nine equal zones using depth bands and horizontal positions on oblique views, or compass directions on vertical views.

Six-Figure Grid Reference

A precise geographical coordinate that identifies a 100-metre map square by stating the sub-zone letter, the easting line and tenths across, followed by the northing line and tenths up.

Eastings

The vertical grid lines on an Ordnance Survey map that increase in value from west to east across the sheet.

Northings

The horizontal grid lines on an Ordnance Survey map that increase in value from south to north up the sheet.

Central Business District (CBD)

The commercial, financial, and retail heart of a town, identifiable on aerial photographs by closely packed multi-storey buildings, colourful shopfronts, and dense traffic.

Greenfield Site

A plot of previously undeveloped agricultural or natural land on the outskirts of an urban area, favoured by developers for its lower purchase cost and expansion space.

Brownfield Site

An abandoned, derelict, or previously developed urban plot that requires clearing, decontamination, or demolition before new construction can begin.

Ribbon Development

The outward growth of an urban area formed by stringing houses and business premises along main approach roads radiating into the countryside.

Nucleated Settlement

A compact settlement pattern where buildings cluster closely together around a central feature such as a bridge, church, market square, or crossroads.

Dispersed Settlement

A rural settlement pattern consisting of isolated farmsteads and cottages scattered across the landscape rather than gathered in villages.

Bypass

A designated relief road constructed around the perimeter of an urban centre to divert through-traffic and heavy freight away from central streets.

Pedestrianisation

The closure of downtown retail streets to motorised vehicles, creating safe, paved shopping corridors reserved exclusively for foot traffic.

Longshore Drift

The zigzag transport of sand and shingle along a coastline, caused by waves approaching the shore at an angle under the influence of prevailing winds.