Photogrammetry FAQ

What is Photogrammetry ? 

Photogrammetry is defined as the art, science and technology of obtaining reliable information about physical objects and environment through the process of interpreting, measuring and recording.

The word photogrammetry has been derived from three Greek words as follows:
Photo = light; gram = write/sketch; metros=measure.

So, photogrammetry is a process of making some measurement on the surface of photo/ using photograph, where a photograph is a sketch made from light. 

What are the applications of Photogrammetry ?
  • To prepare planmetric topographical maps.
  • To determine the space position of ground objects.
  • For acquisition of military intelligence.
  • To classify soil.
  • For the interpretation of geology.
  • Assessment of crop damage due to floods or other natural calamities.
  • To relocate existing property boundaries.
  • In the field of medicine, etc.

What is Map Scale ?

Map scale is defined as the ratio between map distance to earth distance.

Scale can be represented as one of the following: verbal, ratio, or graphic scale.

Verbal Scale
A verbal scale would be a statement found on the map telling a map reader the relationship between the map distance and earth distance. An example would be, "3cm on the map equals 1Km".

Ratio Scale
A Ratio scale is a representative fraction (RF) that expresses one unit of a measurement on the map, in either english or metric units, is equal to the number of the same units on the earth. Example: if 1cm was measured on a map with RF 1:24,000, the earth distance would be equal to 24,000cm which when converted to meters would be equal to 240m; or 1 inch on a map with a scale of 1:24,000, the earth distance would be equal to 24,000inches which when converted to feet would be equal to 2000ft. 
Graphic Scale
A graphic scale is a graphic divided into segments showing the distance on a map labeled with the earth distance it represents.

What is Small Scale Map and Large scale map ?

Small scale maps show large geographic areas, showing less detail. Large scale maps show small geographic areas, showing more detail.

A maps scale will directly influence the size of the phenomena being represented on the map.
For example, a ground area of 300 x 300 meters square would be 4.80 x 4.80 milimeter square on a 1:62,500 scale map (small scale), while the same area would be represented as 12.80 x 12.80 milimeter square on a 1:24,000 scale map (medium scale). In, general, 'the larger the scale the more detail, the smaller the scale the less detail.

What is Aerial Triangulation ?
Aerial Triangulation is ways of confirm and calculate 3dimensional object coordinates by photogrammetric suggests that, by using images exposed from completely different positions, covering constant object. Read More

What is Bundle block Adjustment ?

Bundle block Adjustment is a mathematical technique (triangulation) that determines the position and orientation of each image as they existed at the time of image capture, determine the ground coordinates measured on overlap areas of multiple image, and minimizes the error associated with the imagery, image measurements, and GCP’s. This is essentially a simultaneous triangulation performed on all observations.

What is Terrestrial photograph ?

Terrestrial photographs are taken with ground based cameras. The position and orientation of the camera are often measured directly at the time of exposure. The instrument used for exposing such photograph is called photo theodolite.

What is Aerial photograph ?

Aerial photographs are taken from the air by special carriers mounted in au aircraft flying over the area with the camera axis vertical or nearly so.

What is XYZ File ?

Each record of an XYZ file has a valid Point Label followed by the point's X,
Y and Z coordinates. Sometimes, accuracy estimates for the point in X,Yand Z may also be included, but these are not used by V-STARS. The format for an XYZ file is shown below.
Point Label Point Coordinates (X,Y,Z) Point Accuracies (X,Y,Z)
Examples of some records are given below:
POINT1 (12.5122, -2.2231, 3.4456); 0.0012, 0.0010, 0.0018
REF22 -1.3132 3.5932 5.3768 0.0004 0.0008 0.0003
LINE1 19.5422 -6.8776 2.7896 0.0022 0.0015 0.0005

What is Theodolite ?

A precise angle measuring instrument. It consists of a sighting telescope which can be moved in azimuth and elevation. The azimuth and elevation angels are very precisely encoded. Using two widely separated instruments operators can sight through the telescopes on a common point and produce the 3-dimensional coordinates of the point by triangulation.

What is RMS?

Root-Mean-Square - a statistical measure of the dispersion or spread of a group of data. The name comes from the fact that the RMS is equal to the square Root of the Mean (average) of the sum of Squares of the values in the data set. With regards to resections, the RMS provides a measure of the quality of the resection.

What is Coordinate Reference System?

The coordinate reference system is the backbone of a mapping project. It provides the framework to tie together all field survey and map data. The coordinate reference system must be specified for the final map product. Typically, the State Plane Coordinate zone or the universal transverse Mercator (UTM) zone in which the project is located is used to define a mapping coordinate system. The photogrammetric engineer must be familiar with the reference datum, the coordinate system definition, and the methods required to transform all data into the final map coordinate system.

What is Survey Accuracy Standards?

Ground control should be established to a level of accuracy commensurate with that specified for the final map product. Careful planning and analysis of the basic control and photo control ground surveys should be performed by the contractor to ensure that sufficient accuracy will be obtained throughout the
project area to meet map compilation and aerotriangulation criteria.

a. Accuracy required. Field surveys to establish project control should ideally originate from higher order control established by USACE, NGS, USGS, or local agencies. Field surveys should be designed for the lowest order survey that will meet project requirements, as specified in Table 2-4. As the accuracy standard
increases, the cost of the survey will increase significantly. As a rule of thumb, the accuracy of the horizontal photo control should be twice the horizontal map accuracy to be produced, and the vertical photo control accuracy should be as accurate as the spot elevation accuracy to be produced. The accuracy of a given photo control point is the propagated error in the basic control survey points combined with the error contributed by the survey to connect to the photo control point.

b. USACE standards. USACE standards and guidelines for control surveys are presently under development. Until the USACE standards are available, the FGCC standards discussed in c below should be used to plan and execute the basic and photo control surveys for photogrammetric aerotriangulation and mapping.

c. FGCC standards. Standards and Specifications for Geodetic Control Networks (FGCC 1984) may be used to plan and execute the basic and photo control surveys for photogrammetric aerotriangulation and mapping. FGCC relative accuracy standards for horizontal and vertical control are shown in Tables 5-2
and 5-3. The FGCC document also gives procedural specifications for triangulation, trilateration, traversing, Global Positioning System (GPS) and Doppler positioning, photogrammetry, and leveling field survey methods. These specifications should be followed to ensure that the accuracy standard will be met.

What is cartography?

Cartography is a science and art of designing, constructing and producing maps. It includes every operation fieldwork to final printing. It is also treated as science of communication.

What are the input materials required for any map making?

Topographical map aims at giving true representation of terrain within the limits of scale showing both natural and man-made elements on the earth surface. These elements are identified and shown in correct positions i.e., their location in terms of planimetric situation and elevation is defined.

The entire work will be divided into 3 parts
  • Collection of data
  • Representation of data
  • Final production of maps.
What are the topographical data source ?

Topographical data is collected from the following sources:

1.    Aerial photography
2.    Field survey
3.    Photogrammetry
4.    Photo interpretation
5.    From existing old maps
6.    Satellite imagery

Sometime when we feel that the particular method is not sufficient to collect the topographical data, we adopt two or more methods, so that the data collected should represent terrain correctly.

What are maps? 

A map is graphic representation of selected natural and man-made features of the whole or part of earth surface on a flat sheet of paper on a definite scale.

What are different types of maps with their scale?

Large Scale Map

1.    Cadastral and revenue map          
2.    Topographical map

Small Scale Maps   

1.    Geographical map                    
2.    Atlas                                                      
3.    Wall maps

What are different types of Geographical maps?

  • International map of world series
  • World aeronautical charts
  • State maps
  • India and adjacent countries
  • Political map of India
  • Physical map of India
  • Road map of India
  • Railway map of India
  • Plastic relief map of India
  • Tourist maps
  • Wall maps
Define scale?

The scale of a map may be defined as the fixed proportion, which every distance between the locations of the points on the map bears to the corresponding distances between their positions on the ground. For an example, if 1cm on a map represents a distance of 5 meters on the ground, the scale of map is said to be 1cm=5m. The scale of a map is expressed by a fraction called “Representative Fraction”(R.F).

 Provide fair mapping procedure?
  • Drawing is done in ink and line work is sharp and uniform without breaks.
  • For a multicolor map, a separate drawing of map for original of each color is prepared. This saves time and expense of color separation in the printing office. However, where due to complicated nature of detail, it is difficult to achieve good registration by this method, it is better to draw details of more than one color on one original and to supply a color pattern to enable the printer to do color separation on negatives of combined original.
  • The scale of fair drawing is larger 11/2  times to twice than the scale of communication and to reduce it to publication scale by photography at time of reproduction. 
  • If the drawing originals are to be stored for an indefinite period of future use, no pasting should be done on them. No stick up should be normally be used on polyester based sheet unless the interval of time for making a film negative of positive original is quite sharp and originals need not be retained after that should be done on a separate sheet which may be scrapped after photography.
  • Blue guide image or copy should be followed exactly for position of symbols.
  • Uniformity in drawing of symbols is maintained i.e., every type of detail is shown by a specific symbol drawn in correct size and style.