PHOTOGRAMMETRY

Aerial photographs and images are commonly used for topographic and planimetric mapping projects, and are commonly captured from an aircraft or satellite. Aerial photography and satellite imagery have large geometric distortion that is caused by various systematic and non-systematic factors. The photogrammetric modelling based on collinearity equations eliminates these errors most efficiently, and creates the most reliable ortho-images from raw imagery. Photogrammetry is unique in terms of considering the image forming geometry, utilizing information between overlapping images, and explicitly dealing with the third dimension; elevation.

This short summary, which states the role and function of Photogrammetry department and each panel's activities, is prepared in such a way that can provide an overview on role of the department in the Agency's mission.
Based on EMA's mission this document provides sample data of two types:

• 1:50000 scale topographic map sheets.
• Other large scale maps, which are being done on the basis of user's request.

APPLICATION OF PHOTOGRAMMETRY

• The fundamental goal of photogrammetry is to rigorously establish the geometric relationship between an object and an image and derive information about the object strictly from an image.

• Photogrammetry is required for various applications such as mapmaking, Geology, Forestry, Agriculture, engineering work, environmental study, military intelligence etc. However, the best-known application of photogrammetry is the compilation of topographic maps and surveys complete with contour lines based on measurements and information obtained from aerial photograph and /or space imageries. The compilation usually performed by means of available analogue or analytical instruments. But these days due to the technological advancement in the field the process is transformed to digital workstations. This new method of digital map-making process using computer assisted instruments laid the foundation of Geographic Iinformation System in EMA. The production of digital out put is significant to respond an increasing demand in geo-information at various scales and at the same time developing the new technologies in the organisation. The end product, which is in digital form, will enable users to automate geo -information data.

The photogrammetry department offers several products and services, including:

The perepartion of Topographic maps at various scale

•            Making cadastral maps
•            Verifies landuse classification using field survery

• The provision of DTM for project area
• The coordination and stewardship of aerial photographs in public sector
• The provision of large scale topographic maps for development projects
• Provision of Orthophoto maps for development project planning

• Coordination and supervision of aerial photography mission within the public service

• Consultancy and supervision of cadastral map projects within the public service

     Working Processes

In order to perform its activity the department has proceases:

1. Aerial Triangulation and adjestment
2. Restituttion
3. Orthophoto 
   

•  Aerial triangulation and adjustment

Aerial triangulation is the process of assigning coordinates to a sufficient number of points in the photogrammetric models. Its primary task, processes and determines X, Y and Z ground coordinates of individual points based on measurement from photographs. For this the practical minimum numbers of control points required in each stereo model are three horizontal and four vertical points. Thus, it is possible to identify other minor ground control points photogrammetrically using few actual ground control points collected in the field. Therefore, it can be considered as doing field survey in the office very easily and less costly. The general technical procedures include: -

• Preparation (index map, block diagram, etc.)
• Pass, Tie and control point sellection
• Point transfer and pricking
• Measurement image coordinates
• Adjustment (computation)

• Restitution

Over time, the development of photogrammetry has passed through the phases of plane table photogrammetry, analogue photogrammetry, analytical photogrammetry, and has now entered the phase of digital photogrammetry. All these phases has stamped their own foot print on the restitution stage of photogrammetric map production (feature extraction stage). The Restitution process of EMA has gone through the last three phases of photogrammetry development. The presently working optical or mechanical instruments are used to reconstruct three-dimentional geometry from two overlapping photographs. The main product at this phase is remained to be topographic maps in analogue and digital format.

The traditional, and largest, application of photogrammetry is to extract topographic information such as terrain models from aerial photographs. Photogrammetric techniques have also been applied to process satellite images and close-range images in order to acquire topographic or nontopographic information of photographed objects. The Restitution Process of Photogrammetry department in most of its production time deals with topographic information production and planimetric information production from stereo model of aerial photographs. Topographic information includes spot height information, contour lines, and elevation data. Planimetric information includes the geographic location of buildings, roads, rivers, and so forth.

This is one part of the department where feature extraction is carried out. Feature extraction is a spatial phenomenon at a certain plotting scale that demands high level interpretation and measurement. It is the most time consuming and expensive step. For the quality activity, the true planimetric position and elevation should be maintained. All features have a geometric representation that can be a point, line or an area. Each of which has got their own identification level or layer. The basic data groups include hydrology, vegetation, transport network, relief, special areas and real-estate boundaries. Line maps of various scales are the conventional products in the process  being Photogrammetry is the basic data collection technique. large scales maps produced for urban areas and  other for Engineering work

•  Photo mapping /Orthophoto panel/

In Photogrammetry department, Orthophoto Process is responsible in making digital orthophoto map based on users' specification.

• A digital orthophoto is digital image of an aerial photograph or satellite imagery of the earth surface in which image displacement caused by camera tilt and terrain relief have been removed. Within this process one can obtain the same geometry as a map. They can be used as a supplement to topographic lines maps < this can be refered from the database Index.apr which show areas in Ethiopia covered by orthophoto map as a theme>. It is also used as a back drop images in GIS for further analysis.

• An orthophoto can be used as a map with out cartographic processing or it can be cartographically enhanced and supplemented with lines, names etc to improve its quality.  Orthophoto can be produced faster and cheaper than conventional line maps (Topographic maps).It helps to cover inaccessible areas. 

The data provided should be meclnded

Ortho Photo that combines variety of database of different types has been done in this process. Projects such as Abay-Tekeze, Humera, Shiraro, Addis Ababa and others are prepared from aerial photograph and satellite imageries. These projects took advantage of orthophoto for their immediate demand in drainage feasibility studies, monitoring natural resources, development planning and manageming construction activities.

Digital mapping using workstation to produce ortho images from scanned diapositive has started. AS areult, Users can get the output either in hard copy or soft copy for further analysis. In this regard, many projects such as mekele, Adama, Dire dawa, other small size orthophotes etc. can be mentioned.

• The available Photogrammetric Instruments and Software for Map production

It is clear that observation and measuring image positions and carrying out different photogrammetric tasks need specific photogrammetry equipments. With the advent of computing and imaging technology, photogrammetry has evolved from analogue analytical digital photogrammetry .As a chronological generation the instruments used in the department can be sub-divided in to three categories. For a better clarification, the instrument used in the department and their function is given below.

Instruments data

For a better production, some of the analogue instruments are upgraded in such a way that they can produce maps in a digital format (software-Micro station). For a brief summery, the software used in data processing in the production of Geo-information in the department is given below.