Photogrammetry and Remote Sensing

Remote sensing is the science of obtaining information about an object or area without physical contact, by recording the electromagnetic radiation reflected or emitted from the Earth's surface with sensors on satellites, aircraft or drones. It gives repeated coverage of large and inaccessible areas at low cost.

Applications in civil engineering

• Land use and land cover mapping: classification of urban, agricultural, forest and water areas from multispectral images.
• Route and site selection: alignment of roads, canals, pipelines and transmission lines, and choice of dam, bridge and airport sites using terrain, drainage and geology.
• Water resources: mapping of rivers, lakes and catchments; snow and glacier monitoring; flood extent and flood-plain mapping; watershed management.
• Geology and soil: identification of rock types, faults and lineaments, landslide-prone slopes and soil types for foundation and slope-stability studies.
• Disaster management: assessment of damage after earthquakes (e.g. Gorkha 2015), floods and landslides, and hazard mapping.
• Urban planning: monitoring of urban growth, infrastructure inventory, and change detection.
• Environmental monitoring: vegetation health, erosion, deforestation, pollution of water bodies, and impact assessment of projects.
• Construction and survey: DEM and contour generation, topographic mapping and quantity estimation.

Aerial photography is taken by an aerial camera fixed in an aircraft. Good planning decides cost, accuracy and coverage.

Planning (flight planning)

1. Purpose and scale: the scale of the photographs (and so the map) depends on the use. Photo scale $S = f/H$, which fixes the flying height $H = f/S$ for the camera focal length $f$.
2. Area and boundaries: the project area is marked on an index map and flight lines are drawn parallel to the longer side to reduce the number of turns.
3. Overlap: forward (end) overlap of about 60 % for stereoscopic vision and side overlap of about 25–30 % between adjacent strips.
4. Photo coverage and number: ground coverage of one photo $=$ photo size $\times$ scale denominator; the air base $B = (1-\text{end lap})\times$ ground coverage; the number of photos is computed from the strip length and strip count.
5. Ground control: control points are planned and marked (signalised) before the flight.
6. Season and time: clear sky, little cloud and haze, the sun about 30–40° above the horizon to avoid long shadows, and a season with minimum foliage if the ground is to be seen.

Taking the photographs

• The aircraft flies along straight parallel lines at constant height and speed with the camera axis vertical (tilt less than 3°).
• The camera is operated by an intervalometer so that exposures give the required overlap.
• Crab and drift are corrected by rotating the camera; the exposure, film speed and filters are adjusted for the light.
• The strips are flown, the film is processed, and the photos are indexed and checked for gaps, tilt and quality.

Relief displacement is the radial shift in the position of the image of a point on a vertical photograph caused by its elevation above or below the datum. Points above the datum are displaced away from the principal point (nadir); points below are displaced towards it. On flat ground there is none.

          O  (camera)
         /|\
      f / | \
       /  |  \ H
    a'  a |   \
   -------n-----  photo plane
            \ h  A
 ground   --A0--n'--     datum

Expression

Let A be the top of an object of height $h$ above the datum, A₀ its base on the datum, $H$ the flying height above the datum, and $R$ the horizontal ground distance from the nadir point to A. Let $a$ and $a_0$ be the images of A and A₀, and $r$ and $r_0$ their radial distances from the nadir point (principal point of a vertical photo) on the photo.

By similar triangles in the camera:

Relief displacement $d = r - r_0$:

since $r = fR/(H-h)$. So

Conclusions: displacement increases with the radial distance $r$ and the height $h$ and decreases with the flying height $H$; it is zero at the principal point.

Remote sensing

Remote sensing is the collection of information about the Earth's surface without contact, by sensors on aircraft or satellites that record the electromagnetic energy reflected or emitted by objects.

Types

• Passive remote sensing: depends on the sun's energy (optical and thermal sensors, e.g. Landsat, Sentinel-2, aerial photographs).
• Active remote sensing: the sensor sends its own signal and records the return (radar, SAR, LiDAR).
• By platform: ground-based, airborne and spaceborne. By bands: panchromatic, multispectral, hyperspectral.

Applications in civil engineering

• Land use and land cover mapping: classification of urban, agricultural, forest and water areas from multispectral images.
• Route and site selection: alignment of roads, canals, pipelines and transmission lines, and choice of dam, bridge and airport sites using terrain, drainage and geology.
• Water resources: mapping of rivers, lakes and catchments; snow and glacier monitoring; flood extent and flood-plain mapping; watershed management.
• Geology and soil: identification of rock types, faults and lineaments, landslide-prone slopes and soil types for foundation and slope-stability studies.
• Disaster management: assessment of damage after earthquakes (e.g. Gorkha 2015), floods and landslides, and hazard mapping.
• Urban planning: monitoring of urban growth, infrastructure inventory, and change detection.
• Environmental monitoring: vegetation health, erosion, deforestation, pollution of water bodies, and impact assessment of projects.
• Construction and survey: DEM and contour generation, topographic mapping and quantity estimation.

Remote sensing process

Remote sensing is the collection of information about the Earth's surface without touching it. The process has these stages:

 Sun --> Atmosphere --> Target --> Atmosphere --> Sensor
 (A)       (B)          (C)          (B)          (D)
                                                   |
        Ground station <-- transmission <----------+
              |
        Processing & analysis --> Application

1. Energy source (A): the sun (passive) or the sensor (active) provides electromagnetic energy.
2. Radiation and atmosphere (B): energy travels to the target and back, and is partly absorbed and scattered by the atmosphere.
3. Interaction with the target (C): the surface reflects, absorbs or emits energy according to its spectral signature (e.g. vegetation, water, soil).
4. Recording by the sensor (D): satellite or aircraft sensors record the energy as images.
5. Transmission and reception: the data are sent to a ground station.
6. Processing and interpretation: corrections, enhancement and classification; visual or digital interpretation.
7. Application: the information is used in maps, reports and decisions.

Applications in civil engineering

• Land use and land cover mapping; urban growth monitoring.
• Route and site selection for roads, canals, dams, bridges and pipelines.
• Water resources, flood mapping, catchment and watershed studies.
• Geological and landslide hazard mapping, soil mapping.
• Disaster assessment after earthquakes and floods.
• DEM and contour generation; environmental impact studies.

Remote sensing records information about the Earth's surface from a distance using electromagnetic energy.

Types of remote sensing

1. By energy source
   • Passive: uses natural energy, mainly reflected sunlight or emitted thermal radiation (e.g. Landsat, Sentinel-2, aerial photography).
   • Active: the sensor sends its own energy and records the return (e.g. radar/SAR, LiDAR).
2. By platform: ground-based, airborne (aircraft, drones) and spaceborne (satellites).
3. By wavelength region: visible and near-infrared, thermal infrared and microwave remote sensing.
4. By number of bands: panchromatic, multispectral and hyperspectral.

Applications

• Land use and land cover mapping: classification of urban, agricultural, forest and water areas from multispectral images.
• Route and site selection: alignment of roads, canals, pipelines and transmission lines, and choice of dam, bridge and airport sites using terrain, drainage and geology.
• Water resources: mapping of rivers, lakes and catchments; snow and glacier monitoring; flood extent and flood-plain mapping; watershed management.
• Geology and soil: identification of rock types, faults and lineaments, landslide-prone slopes and soil types for foundation and slope-stability studies.
• Disaster management: assessment of damage after earthquakes (e.g. Gorkha 2015), floods and landslides, and hazard mapping.
• Urban planning: monitoring of urban growth, infrastructure inventory, and change detection.
• Environmental monitoring: vegetation health, erosion, deforestation, pollution of water bodies, and impact assessment of projects.
• Construction and survey: topographic mapping, DEM and contour generation.

Field procedure of aerial photogrammetry

1. Planning: decide the purpose, scale and accuracy; choose the camera, flying height and overlaps (60 % end lap, 25–30 % side lap); prepare the flight plan.
2. Ground control: establish horizontal and vertical control points (by traversing, levelling or GPS) that can be identified on the photos, and signalise them with targets before the flight.
3. Photography: fly the strips at the planned height and speed in suitable weather; the camera exposes at fixed intervals.
4. Processing: develop the film (or download digital images), prepare contact prints and a photo index.
5. Photo-control identification and plotting: pass the control to the photos, orient the stereo pairs in a plotter, and draw the map or compute coordinates and contours.
6. Field completion and checking: annotate names and features, check the map in the field.

Relief displacement

It is the radial shift of the image of a point on a vertical photo, due to its height $h$ above the datum. From similar triangles,

where $r$ is the radial distance of the image from the principal point and $H$ is the flying height above the datum. High points move outward from the principal point, low points move inward, and the displacement is zero at the principal point. Because of this, a photograph of undulating ground is not to scale and cannot be used as a map unless corrected (by rectification or orthophoto production). It also lets heights of objects be found: $h = dH/r$, and it gives the parallax needed for stereoscopic viewing.

Types of aerial photographs

1. By camera axis:
   • Vertical: the camera axis is vertical (tilt less than 3°); used for mapping.
   • Tilted (low oblique): axis tilted 3° or more but the horizon is not visible.
   • Oblique: axis intentionally tilted; the horizon is visible in high oblique photographs.
2. By scale: large, medium and small scale photographs.
3. By film: panchromatic, infrared, colour and false-colour photographs.

Scale of a vertical photograph

        O (lens)
       /|\
    f / | \
     /  |  \
   a----n----b   photo
   -----------
   A----N----B   ground (H - h below O)

The scale is the ratio of a distance on the photograph to the corresponding distance on the ground. By similar triangles $Oab$ and $OAB$:

where $f$ is the focal length, $H$ the flying height above datum, and $h$ the elevation of the ground above datum. For flat ground at datum, $S = f/H$. For undulating terrain, an average scale uses the mean elevation $h_{av}$: $S = f/(H-h_{av})$. The scale also equals $S = \dfrac{\text{photo distance}}{\text{map distance}}\times\text{map scale}$.

Scale of a vertical photograph

The scale of a vertical photograph is the ratio of a distance on the photo to the same distance on the ground. By similar triangles in the camera,

where $f$ is the focal length of the camera, $H$ is the flying height above the datum and $h$ is the elevation of the ground above the datum. The scale varies from point to point when the terrain is not flat, and is larger for higher ground. If $h = 0$, $S = f/H$.

Uses of photogrammetry

• Preparation of topographic maps and contour maps, and orthophotos.
• Route location for roads, railways, pipelines and canals.
• Site selection for dams, bridges and airports.
• Measuring volumes (earthwork, stockpiles) and cross-sections.
• Mapping of inaccessible, hazardous or very large areas.
• Land use, forestry, geology and urban planning surveys.
• Cadastral and property mapping; disaster damage assessment.
• Close-range photogrammetry: structural deformation, monuments and archaeology.

Principle of stereoscopy

Stereoscopy is the ability to see in three dimensions. A person has two eyes about 65 mm apart (eye base). Each eye views an object from a slightly different angle, so the two retinal images differ; the brain fuses them into a 3D impression and judges depth from the parallax between the images.

In photogrammetry, two overlapping photographs of the same area taken from different camera stations (air base $B$) serve as the "two eyes". When the left photo is seen by the left eye and the right photo by the right eye (using a lens or mirror stereoscope, or by anaglyph/polarised displays), the overlap is seen as a three-dimensional model. Height differences are measured from the differences in parallax: $h = \dfrac{H\,\Delta p}{p + \Delta p}$ (where $p$ = absolute stereoscopic parallax).

   O1 <------ B ------> O2     two exposures
    \                 /
     \               /
      \             /
       [ overlap ]  -> 3D model

Importance of aerial mapping

• Quick preparation of maps of large and inaccessible areas.
• Uniform accuracy and a permanent record of the ground at the time of the flight.
• Cheaper and faster than ground survey for large areas.
• Gives contours, spot heights, planimetric details and orthophotos for planning and design of roads, dams and cities.
• Allows updating of maps and monitoring of change (floods, landslides, urban growth).

Photogrammetry is the science and art of obtaining reliable measurements and maps of objects and terrain from photographs. Its main branches are aerial (terrestrial-base) and close-range photogrammetry.

Importance

• Provides fast, accurate maps of very large areas at a lower cost than ground survey.
• Gives a permanent, objective record of the ground that can be re-measured at any time.
• Makes measurement possible in inaccessible, dangerous or restricted places.
• Produces many outputs from one set of photos: planimetric maps, contour maps, DEMs and orthophotos.
• Reduces field work, so projects are finished in less time.

Uses

• Topographic and large-scale engineering mapping.
• Route and site investigation (roads, railways, canals, dams, transmission lines).
• Volume computation of earthwork and quarries.
• Cadastral surveys and land-use planning.
• Geology, forestry, agriculture and urban planning.
• Structural deformation, monument and archaeological documentation (close range).
• Disaster damage assessment and flood mapping.

The scale of a photograph is the ratio of a length measured on the photograph to the corresponding length on the ground.

        O  (exposure station)
       /|\
    f / | \
     /  |  \
   a----n----b    photo plane
        |
   H-h  |
        |
   A----N----B    ground at height h
   -----------    datum

Derivation

Let O be the lens centre, $f$ the focal length, $H$ the flying height above datum, and A and B two ground points at elevation $h$ above the datum, whose images are $a$ and $b$ on a vertical photo. The line $ab$ on the photo is parallel to $AB$ on the ground, as the camera axis is vertical.

Triangles $Oab$ and $OAB$ are similar. The perpendicular height of O above the plane of $ab$ is $f$ and above the ground plane AB is $(H-h)$. Hence

Notes

• For flat ground at the datum ($h = 0$): $S = f/H$.
• If the ground is not level, the scale is different for each elevation; the average scale uses the mean ground elevation $h_{av}$: $S_{av} = f/(H-h_{av})$.
• Scale can also be found from the map: $S = \dfrac{\text{photo distance}}{\text{map distance}}\times\text{map scale}$.
• Example: $f = 152$ mm, $H = 3000$ m above a ground at 500 m gives $S = 0.152/2500 = 1:16\,447$.

Relief displacement is the radial displacement of the image of a point on a vertical photograph due to its elevation above or below the datum plane. On a vertical photograph the displacement is measured outward from (for high points) or inward towards (for low points) the principal point, which is also the nadir point.

            O (lens)
          / | \
         /  |  \          H = flying height above datum
        /   |   \
   a0  a    n               photo (r0, r)
   ----+----+---
                \   h
      A0     A    N0  (object top A above base A0)
   ============ datum

Explanation with the sketch

• Let a vertical object AA₀ stand on the datum, with its top A at height $h$. Its base A₀ and top A lie on the same vertical line, which is a line through the nadir point N.
• On the photo, the base A₀ forms image $a_0$ at radial distance $r_0 = fR/H$ from the nadir point $n$, and the top A forms image $a$ at the larger distance $r = fR/(H-h)$.
• The vertical object therefore appears as a radial line $a_0a$ pointing away from $n$: the "lean" of tall buildings, towers and trees on aerial photographs.

Expression

• $d \propto r$: no displacement at the centre, maximum at the corners.
• $d \propto h$: taller objects are displaced more.
• $d \propto 1/H$: higher altitude flights reduce displacement.

Uses

Height of an object: $h = \dfrac{dH}{r}$. Relief displacement also creates parallax for stereoscopic viewing and has to be removed by rectification to make orthophotos.

Satellite imagery is an image of the Earth recorded by sensors on satellites (e.g. Landsat, Sentinel, SPOT, IRS, WorldView) in different wavelength bands, usually in digital form.

Uses

• Land use / land cover mapping and change detection.
• Topographic and thematic mapping, DEM generation and urban planning.
• Monitoring of floods, landslides, glaciers, forest fires and drought; damage assessment after earthquakes.
• Selection of sites and routes of roads, canals, dams and pipelines.
• Water resources: rivers, lakes, snow cover and catchment studies.
• Agriculture and forestry: crop condition, yield estimates, and forest cover.
• Environmental monitoring of pollution, erosion and deforestation.

Advantages

1. Covers large and inaccessible areas in one image (wide synoptic view).
2. Repeat coverage at regular intervals allows monitoring of change over time.
3. Multispectral (and thermal / microwave) bands give information beyond what the eye sees.
4. Digital data is easy to process and combine with GIS.
5. Cheaper and faster than aerial or ground survey for large areas.
6. Many images are free or low-cost (Landsat, Sentinel), and the archive provides historical data.
7. Uniform data over political boundaries, and safe over dangerous areas.

A tilted photograph is an aerial photograph taken with the camera axis unintentionally inclined from the vertical by more than 3° (tilt up to about 3° is regarded as a vertical photograph). Its scale varies across the photo and it needs rectification before it can be used for mapping.

Side overlap (side lap) is the overlap between photographs of adjacent flight strips, measured at right angles to the direction of flight; it is normally 25–30 % so that no gaps are left between strips.

The principal point of a photograph is the foot of the perpendicular from the lens (perspective centre) to the photo plane, i.e. the point where the optical axis of the camera meets the photograph. It is located at the intersection of the lines joining opposite fiducial marks.

Flying height is the vertical distance of the aircraft (the camera lens at the moment of exposure) above a given datum, usually mean sea level, or above the average ground level. Photo scale is $S = f/H$ for flat ground, so for a given camera a greater flying height gives a smaller scale.

Photogrammetry is the science and art of obtaining reliable measurements, maps and three-dimensional information of objects and terrain from photographs. Aerial photogrammetry uses photographs taken from aircraft or drones, and close-range photogrammetry uses ground-based cameras.

   Camera stations O1 -------- O2
        \                      /
         \    overlap area    /
          \__________________/
              ground  -> 3D model -> map

Basic steps

1. Plan the flight and establish ground control.
2. Take overlapping photos (60 % end lap, 25–30 % side lap).
3. Process the photos, orient the stereo pairs, and measure.
4. Produce maps, DEMs, orthophotos or coordinates.

Uses

• Topographic, planimetric and contour maps of large areas.
• Engineering projects: route location, site selection for dams, bridges and airports.
• Earthwork volume and cross-section measurement.
• Cadastral surveys, land use planning and urban planning.
• Geology, forestry, agriculture and archaeology.
• Disaster management: flood and landslide mapping, damage assessment.
• Close-range work: structural deformation, monuments, accident reconstruction.

Limitations

• Costly equipment, aircraft and trained personnel; uneconomical for small areas.
• Weather dependent: clouds, haze and shadow affect photo quality.
• Ground control is still needed, and field verification of names and details is necessary.
• Dense vegetation, shadows and tall buildings hide the ground; small details may not be visible.
• Accuracy depends on scale, camera quality, tilt and relief displacement; processing needs skill.
• Boundaries and underground features cannot be seen.

The photo scale is found from a map distance, and then the flying height from $S = f/(H-h)$.

Step 1: Ground length of PQ

Map scale 1:50 000, so PQ on the ground:

Step 2: Photo scale

Step 3: Flying height

Answer: The photo scale is 1:15 000 and the flying height of the aircraft is 3950 m above datum (3600 m above the average ground).

Considerations in flight planning

• Purpose, scale and accuracy: decide the photo scale $S = f/H$, the camera focal length and the flying height $H = f/S$.
• Area and flight lines: draw parallel strips, usually along the longer side, to reduce turns.
• Overlap: end lap about 60 % and side lap about 25–30 %.
• Air base and number of photos: computed from the ground coverage per photo and the overlaps.
• Ground control: planned and signalised before the flight.
• Weather and season: clear sky, no haze, minimum cloud shadows; sun angle of about 30–40°; low foliage season if the ground is to be seen.
• Aircraft and camera: speed, endurance, stability, tilt below 3°, and navigation (GPS).
• Cost and time: economic block layout to minimise flying time.

Merits of photogrammetry

• Fast and economical for large areas.
• Gives a permanent record and uniform accuracy.
• Maps inaccessible or dangerous areas safely.
• Many products from one set of photos: maps, DEMs, orthophotos.

Limitations

• Costly equipment and trained staff; not economical for small areas.
• Weather dependent; hidden areas under trees, shadows and buildings.
• Ground control and field checking are still required.
• Accuracy is affected by tilt, relief displacement and camera quality.