12 DIGITAL PHOTOGRAMMETRY

Ms. Kaushal Panwar

epgp books

 

 

 

 

Learning Outcome

  • Student will get to know about digital photogrammetry.
  • Student will acquire skill to work upon DEM, DTM and ortho photos.
  • Student will be equipped with knowledge to study further digital photogrammetry needs, applications and advancement in remote sensing field.

 

Introduction to Photogrammetry:

  • Photogrammetry as a science is among the earliest techniques of remote sensing.
  • The word photogrammetry is the combination of three distinct Greek words: ‘Photos’ – light; ‘Gramma’ -to draw; and ‘Metron’ –to measure. The root words originally signify “measuring graphically by means of light.”
  • The fundamental goal of photogrammetry is to rigorously establish the geometric relationship between an object and an image and derive information about the object from the image.
  • For the laymen, photogrammetry is the technological ability of determining the measurement of any object by means of photography.

 

Why Digital Photogrammetry?

  • With the advent of computing and imaging technology, photogrammetry has evolved from analogue to analytical to digital (softcopy) photogrammetry.
  • The main difference between digital photogrammetry and its predecessors (analogue and analytical) is that it deals with digital imagery directly rather than (analogue) photographs.
  • Digital photogrammetry invovles processing of imagery of all types, including passive (e.g., optical sensing) or active (e.g., radar imaging), and taken from any platform (e.g., airborne, satellite, close range, etc.).
  • The unique advantages of Digital Photogrammetry in terms of precision and accuracy offers opportunities for automation of DEM/DTM and integration of images acquired on a multi-platform and multi-sensor basis.

 

Techniques of photogrammetry:

  1. Depending on the lenses setting:

           a. Far range photogrammetry (with camera distance setting to indefinite).

       b. Close range photogrammetry (with camera distance settings to finite values).

  1. on the basis of type of surveying:

            a. Terrestrial or ground photogrammetry.

        b. Aerial photogrammetry.

 

A. Terrestrial or Ground Photogrammetry:

 

In terrestrial photogrammetry maps are prepared from terrestrial (or ground) photographs or terrestrial photogrammetry employees take photographs from different points on the earth surface for measurement purposes.

 

The terrestrial photographic surveying considered as the further development of plane table surveying.

 

B. Aerial Photogrammetry:

 

In aerial photogrammetry maps are produced from air photographs (photographs taken from the air).

 

Fig 1: Aerial Photography

 

 

Aerial Photogrammetry encompasses two major areas of specialization:

  • Metrical
  • Interpretive

     The first area is of principal interest to surveyors since it is applied to determine distances, elevations, areas, volumes, cross-sections and to compile topographic maps from measurement made on photographs.

 

Interpretive photogrammetry involves objects from there photographic images and judging their significances. Critical factors considered in identifying objects of shape, sizes, patterns, shadow.

 

Outputs

 

1. Determining the scale of a vertical photograph and estimating horizontal ground distances from measurements made on a vertical photograph.

 

Scale: the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground (i.e. 1 unit on the photo equals “x” units on the ground).

 

If a 1 km stretch of highway covers 4 cm on an air photo, the scale is calculated as follows:

 

Fig 2: Scale of aerial photograph.

2. To determine the equivalent areas in a ground coordinate system using area measurements made on a vertical photograph.

 

The only difference is that whereas ground distances and photo distances vary linearly , ground areas and photo areas vary as the square of the scale (S).

 

Ground area=Photo area * 1/(S*S)

 

3. Determination of object heights from relief displacement measurement.

 

Relief displacement: The images of the tops of objects appearing in a photograph are displaced from the images of their bases this is known as relief displacement. The magnitude of relief displacement depends on:

 

Flying height: When the flying height increased, the relief displacement will be increased.

 

The distance from the object from the nadir point: When the distance of object is more from nadir point, the relief displacement will be more.

 

The height of the feature: When the distance of objects from the nadir point is remain same. But the object height increased or decreased. Higher object is more displaced.

 

Focal Length: When the focal length of camera lens is increased, the relief displacement will be more.

 

Relief displacement is expressed mathematically as:

 

d = hr/H                         (1)


Where, d = Relief Displacement h = Height of the object r = Radial distance from nadir point H = Total altitude of the camera or flying height

 

from equation (1)

 

Height of the object(h) = dH/r

 

Fig 3: Measurement of height of the object from relief displacement

 

4.Determination of object heights and terrain elevations by measurement of image parallax.

 

Image parallax: The term parallax refers to the apparent change in relative position of stationary objects caused by a change in viewing position.

 

The absolute stereoscopic parallax is the algebraic difference, in the direction of the flight, of the distance of the two images of the object from their respective principal points. The parallax difference can be used to determine the height of the objects and the dip and slope from the stereo pairs.

 

To measure the height of an object above or below a reference point from stereo-pair of aerial photograph following data is required: –

 

i)  Flying height above the reference point.

ii)  Photo base – Which can be measured from the stereo-pair

iii)    Parallax difference – It is measured by the use of a set of measuring marks (sometimes called locating marks.

 

Now by using following (parallax) formula the height of the objects can be measured.

 

h = (Z*∆P) / (b + ∆P)

 

For smaller heights e.g. trees, embankments, buildings the formula is further simplified to:

 

h = (Z*∆P) / b

 

Where h = height of object , Z = flying height above the reference point, b = photo base, ∆P = Parallax difference.

 

This formula gives correct result when the photographs are truly vertical.

 

5. Generation of maps in stereoplotters.

 

Stereoplotter: Two pictures of a particular area are simultaneously taken, but from slightly different angles. The overlapping area of the two resulting photos is called a stereo pair. A Stereo plotter is an instrument designed for the production of topographic maps from stereo pairs. With this topographic contours can be plotted on the map and the height of vertical features appearing in the model can be determined.

 

With the advent of technologies Stereo plotters have changed. Starting from projection stereoplotters to Kelsh Plotter to analog stereoplotters to analytical stereoplotter.

 

Analytical stereoplotter is now being used for different analysis. It incorporates a computer which does the work of mathematically aligning the images so that they line up properly. The analytic stereoplotter also allows for storing the data and redrawing at any desired scale.

 

Examples of Analytical Stereoplotters

 

Galileo Digicart

Intergraph Intermap

Kern (now Leica) DSR series

Wild (now Leica) AC and BC series,

Zeiss Planicomp P series

Matra Traster series

Leica Photogrammetric suite (LPS).

 

Fig: Kelsh stereoplotter                                                 Fig: Alpha 2000 stereoplotter

 

6. Generation of Digital Elevation Models and Orthophotographs.

 

Digital  Elevation  Model:  A digital  elevation  model (DEM)  is  a  digital  model  or  3D representation of a terrain’s surface. Three different terms are frequently being used i.e. digital elevation model (DEM), digital terrain model (DTM) and digital surface model (DSM) in scientific literature. In most cases the term digital surface model represents the earth’s surface and includes all objects on it. In contrast to a DSM, the digital terrain model (DTM) represents the bare ground surface without any objects like plants and buildings.

Fig 4: Surfaces represented by a Digital Surface Model include buildings and other objects. Digital Terrain Models represent the bare ground.

 

DEM is often used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface.

Fig 5: Digital elevation model of Orange river.

 

Orthophotos and Digital Orthophotography: An orthophoto is an aerial photograph that has been geometrically corrected or ‘ortho-rectified’ such that the scale of the photograph is uniform.

 

Orthophotos are photographs that have been corrected for distortions due to tilting of the camera during the photographic survey, distortions from the camera lens, and relief distortions.

 

Orthophotos display all the valuable information of a photograph, but unlike a photograph, true distances, angles and areas can be measured directly (Rossi,2004).

 

An orthophoto is an accurate representation of the Earth’ s surface. Orthophotos have the benefits of high detail, timely coverage combined with the benefits of a map including uniform scale and true geometry.

Fig 6: Orthographic views project at a right angle to the data plane. Perspective views project from the surface onto the datum plane from a fixed location.

 

Digital Orthophotograph:

 

A digital orthophoto is a digital image of an aerial photograph with displacements caused by the camera angle and the terrain removed. It, thus, combines the image characteristics of a photograph with the geometric qualities of a map.

 

Requirements:

 

Photogrammetry is the technique of determining the geometric properties of objects from photographic images where the 3D coordinates of points, features or objects can be determined by measurements made in stereopair using the principles of triangulation. The following are essential elements required to produce a digital orthophoto:

 

1) Photo identifiable ground control points.

2) Camera calibration and orientation parameters.

3) A digital elevation model (DEM).

4) A digital image produced by scanning an aerial photograph with a precision high-resolution scanner.

5) Softcopy Photogrammetric Workstations: Processing the imagery to derive image and vector products using Digital Photogrammetric Workstation (DPW).

 

A DPW combines computer hardware and software i.e., a graphics workstation with, in most but not all cases, a stereo viewing device and a 3-D mouse. Software configuration includes Erdas Imagine with Leica Photogrammetric Suite (LPS)

 

For modern DPWs, there’s no specific requirement for the host computer. Often a DPW can be built on a high-end desktop PC with at least 256RAM, one or two 19- or 21-inch monitors and a high-performance graphics card.

 

Ortho Image generation using Analytical Stereo plotter i.e. Leica Photogrammetric suite:

 

Leica Photogrammetric suite (LPS): IMAGINE Photogrammetry (formerly LPS and Leica Photogrammetry Suite) is a software application for performing photogrammetric operations on imagery and extracting information from imagery. IMAGINE Photogrammetry is significant because it is a leading commercial photogrammetry application that is used by numerous national mapping agencies, regional mapping authorities, various DOTs, as well as commercial mapping firms. Aside from commercial and government applications, IMAGINE Photogrammetry is widely used in academic research. Research areas include landslide monitoring, cultural heritage studies, and more.

 

Ortho Image generation involves the following steps to be followed:

Fig 7: Flowchart for orthoimage generation

 

In the course of a photogrammetrical project we can provide:

 

GPS Control

 

Aerial Photography

 

Airborne GPS and Triangulation Triangulation & Adjustments

 

Precise Photogrammetric Observations Contour and Feature Mapping

 

Using a computer called a stereoplotter, the stereo pair can be viewed as a single image with the appearance of depth or relief. Ground control points are established based on ground surveys or aerial triangulation and are viewed in the stereoplotter in conjunction with the stereo pair. In this setting, the image coordinates of any (x,y,z) point in the stereo pair can be determined and randomly selected and digitized. These points, in conjunction with the control points, comprise the data points for the DTM. The accuracy of the final digital orthophoto will depend in large part on the point density of the DTM.

 

 

Fig 8: image before(a) and after(b) Orthorectification.

 

 

Advantages of Photogrammetry

 

Cover areas quickly. Low costs.

 

Easy to obtain/access information from air. Illustrates great detail.

 

3D Visualization.

 

Applications of Photogrammetry:

 

Photomapping Services provides solutions to the highly specialised needs of today’s world to compile datasets from photogrammetry such as the following:

 

Land Surveying: Surveying or land surveying is the technique, profession, and science of determining the terrestrial or three-dimensional position of points and the distances and angles between them. it involves land use land cover planning urban planning wasteland mapping, etc.

Fig 9: Area demarcation for land surveying

 

Topographic mapping: One of the most widespread use of Photogrammetry is topographical mapping, which is considered the primary approach to GIS base data collection and updating. it provides topographical information of the area.

Fig 10: Contours draped over the topography of the area

 

Terrain Models: Digital Elevation Models (DEMs) / Digital Terrain Models (DTMs), Spot Heights, Contours, and Breaklines, which in turn useful to:

 

superimposed over an orthophoto.

 

useful to determine irrigation requirements useful to determine drainage requirements ideal for rural property mapping

 

Fig 11: Contour superimposed on orthophotos.

Fig 12: 1m. DEM extracted from Stereo IKONOS satellite image data.

 

Engineering design:

 

3D model of a busy intersection and railway junction. useful for urban infrastructure planning. useful for infrastructure design and modelling.

 

Fig 13: Engineering design planning

 

Natural resource and environmental inventory: Photogrammetric products are being used to monitor natural and cultural ecosystems.

 

Hydrographic survey:

 

Detailed Drainage Studies

 

3D model of extensive drainage network useful to optimize drainage systems

 

ideal for high density urban mapping

fig 14: A three-dimensional view of the flooded area showing stream centerlines and elevation

 

Geological Mapping: This technique allows to obtain a highly accurate 3D picture of the visible outcrop. The spatial pattern of joints in nature can be investigated using the software. This might help to understand how physical rock properties influence the spatial complexity of fracture systems and develop constitutive scaling relationships for certain rock types.

Fig 15: Lithological map draped over the topography

 

 Medical Photogrammetry: Photogrammetric measurement for the diagnosis and treatment of human conditions and for bio-medical research is a form of close-range photogrammetry with its own distinctive challenges and constraints. Reports of fully digital systems are somewhat limited so far. Conventional photogrammetric techniques have been used for a broad variety of medical applications. Amongst them, craniofacial mapping, human trunk and extremity mapping, wound and sore mapping, as well as dental mapping are the most common. Nowadays, laser scanning and 3D probing devices have superseded photogrammetry for many of these applications, whilst photogrammetry has advanced to on-line measuring and real-time navigating applications in the medical field.

fig 16:Bone structure and 3D representation of skin

 

Police investigation: There’s an easier way to get all the information at an accident scene than walking around taking painstaking measurements – by using photogrammetry to map the scene instead. Your camera can help you gather all of the important information quickly and accurately, so that you can clear out faster and get traffic back to normal.

 

Fig 17 collision analysis and accident reconstruction

 

Architectural photogrammetry: Photogrammetric techniques are being used for the representation of the facades or elevations of historic buildings and structures. The most common product is the line drawing which delineates architectural form. Such surveys are needed by the various disciplines involved in building repair and conservation.

 

Fig 18: Final graphic documentation of the churches of Ceadea (top) and Trabazos (bottom): ground plan, orthogonal profiles with photographic texture and photorealistic 3D model.

 

Archaeological mapping: Using Terrestrial photogrammetry archaeologists can produce photographic plans of sites and their stratigraphy, take accurate measurements directly from the photo, and import photographic data into other computerized technologies for mapping and visualizing archaeological features. The production of a photogrammetric image involves the combination of a number of technologies:

 

Total station surveying.

 

Traditional archaeological photography. Geospatial rectification.

 

By combining these technologies, we are able to produce a hybridized documentation technique that can serve many purposes.

 

Fig 19: 3D model of archaeological site

 

Industrial machinery: Photogrammetry has been increasingly applied as a precise 3D measuring tool in industrial and engineering works. Analytical photogrammetry is now routinely employed in tasks of measurement:

 

o   Machine tool inspection.

 

o   Fixture checking.

 

o   Structural deformation monitoring.

 

o   Provision of control databases to guide

 

o   Industrial robots

 

o   Measurement of structures in earth orbit.

 

photogrammetric method has advantages of a non-contacting 3D object reconstruction by means of spatial rays. It provides, a short recording time on-site nearly independent from the amount of object points to be measured, and the possibility to choose the recording stations in a very flexible way, If a dynamic or kinematic process has to be recorded, photogrammetry seems to be the only way to measure a whole object simultaneously.

Fig 19: 3D CAD design

 

Military applications: Mosaic is a aerial photograph of a large area, made by carefully fitting together aerial photographs of smaller areas so that the edges match in location, and the whole provides a continuous image of the larger area. Mosaics are intensely being used by military projects because they provide:

 

•  Synoptic view all over the target area.

 

•  Target planning.

 

•  Topography of the area.

 

•  Decision making, Etc.

Fig 12: Mosaic generated for military Planning.

 

 

you can view video on DIGITAL PHOTOGRAMMETRY

 

References

  1. http://www.aabsys.com/insights/articles/basics-digital-photogrammetry
  2. www.msu.ac.zw/elearning/material/1158675019Digital%20Photogrammetry.doc
  3. https://nrsc.gov.in/sites/all/pdf/Digital%20Photogrammetry2016-new.pdf
  4. http://www.aboutcivil.org/photographic-surveying-
  5. photogrammetry.htmlhttp://ijsetr.org/wp-content/uploads/2014/10/IJSETR-VOL-3-ISSUE-10-2800-2805.pdf
  6. http://shaileshchaure.com/Notes/IMAGEPARALLAX.pdf
  7. https://en.wikipedia.org/wiki/Stereoplotter
  8. https://www.fig.net/resources/proceedings/fig_proceedings/fig2006/papers/ps05_08/ps05 _08_02_ayhan_etal_0552.pdf
  9. https://pubs.usgs.gov/unnumbered/70039180/report.pdf
  10. https://www.researchgate.net/publication/228666730_New_developments_in_medical_ph otogrammetry
  11. http://www.mdpi.com/2072-4292/6/6/5671/htm
  12. http://www.jezreelvalleyregionalproject.com/practical-uses-for-photogrammetry-on-archaeological-excavations.html
  13. http://www.slac.stanford.edu/econf/C930928/papers/018.pdf
  14. http://www.ce.utexas.edu/prof/maidment/grad/tate/research/orthophotos.html
  15. http://www.photomapping.com.au/digital-orthophoto