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
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
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
In order to perform its activity the department has proceases:
Aerial Triangulation and adjestment
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
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
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.
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.