Geometric Design Manual-2002
Ethiopian Roads Authority
This chapter presents the survey requirements associated with the geometric design process.
Survey data for design purposes consists of mapping of sufficient detail for the level of
design being undertaken.
In some instance a digital terrain model (DTM) for use with
computer design software may be required.
The survey data product is dependent on project type and can be collected by aerial
photography, field topographical survey, or a combination of the two.
The following factors should be considered when determining the survey data required:
Size and scope of the project
Time requirements to move from data collection to the start of design
Estimated data collection costs
Level of accuracy and detail needed
The project designer is responsible for identifying the appropriate survey data requirements
(type of data, accuracy, area of coverage). The project designer is also responsible for
obtaining the survey data and for selecting the method of data collection.
Method of Data Collection: Photogrammetry vs. Field Survey
Topographical ground survey has the capability of achieving greater accuracy than
photogrammetry. The effectiveness of aerial photography depends on location (urban or
rural), ground cover, etc.
Photogrammetry is sufficiently accurate for most applications and can be more cost effective
for all but small projects. For mapping and DTMs, photogrammetry is usually the preferred
choice. However, if a project is short, has dense foliage, or requires only mapping of limited
features, a field survey is the logical choice. Some fieldwork will be required for most
projects to compile property lines, right-of-way (ROW) information, utility, and culvert,
tree, building, bridge and sign data unavailable through aerial photography.
Elevations of photogrammetric DTM points on hard surfaces are accurate to within ±60
millimeters. If more precise vertical accuracy is required for areas of a project, the data must
be obtained through a field survey. If precise vertical accuracy is required, such as for
highway pavement elevations, or if obstructed views occur, photogrammetric data can be
supplemented with survey elevations. It is recommended that survey data be collected before
the photogrammetric data to help assure the accuracy of the DTM. Table 4-1 provides
guidelines for when photogrammetry, survey, or a combination of both should be used. It
must be noted that this table is a guideline only, and that appropriate methods also depend on
factors such as project location (rural or urban), and length.