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04 - Chapter-4

# 04 - Chapter-4 - Geometric Design Manual-2002 Chapter 4...

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Chapter 4 Geometric Design Manual-2002 Survey Requirements Ethiopian Roads Authority Page 4-1 4 S URVEY R EQUIREMENTS 4.1 Introduction 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: 1. Size and scope of the project 2. Time requirements to move from data collection to the start of design 3. Estimated data collection costs 4. 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. 4.2 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.

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Chapter 4 Survey Requirements Geometric Design Manual-2002 Page 4-2 Ethiopian Roads Authority 4.3 Survey Data Products While survey data requests will typically originate from the unit responsible for the design, they should also serve the requirements of Construction. The project designer has the
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04 - Chapter-4 - Geometric Design Manual-2002 Chapter 4...

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