The defined three dimensional path of a road or route is the alignment. It is designed as 'horizontal' and 'vertical' alignment because of 'plane surveying' and two dimensional drawing. The ideal alignment is a great circle from the start, or ORIGIN to the end or DESTINATION. As long as walking speeds were the norm, alignment was mostly effected by acceptable terrain and directness of route.

Route location technology is very old. Modern route location, design and construction began in the 1800's with the builing of the railway network. Railways were really the first routes with sustained speeds higher than a running horse or man. Railways require precision track location for stability & comfort at limiting speeds. Railways are best with low slope track. Slopes must be within allowable power and traction limits of the locomotive. Curves must be within the tracking and stable limits of the vehicles. In rolling terrain these factors result in a sinuous alignment with many horizontal curves. The costs of construction are related to the amount of earthwork required.

The surveying instruments that provided the precision required were:

1. The 'transit' (horizontal angles to at least the nearest minute).
2. The 'steel chain' (tape) that was dimensionally stable in a large range of weather conditions, and could measure distance to at least .01 feet in 100.
3. The 'level' and
4. the 'rod' that allowed vertical surveying and alignment to a precision of at least .01 feet in 300.

The curviture of the earth is overcome by confining measurement to a radius of 300 feet from the level. Techniques borrowed from 'Land Surveying' allowed the required precision to be extended over very long distances.

For successful operation of railways and roads long distance prcision is not required. What is required is precision over the distance base where changes in direction take place. This keeps vertical and horizontal accelerations within limits acceptable to vehicles and cargo. The vehicles must be stable and controllable from zero to maximum speed. The cargo includes passengers and goods. The limits are usually defined for comfort or damage. Any part of the cargo, occupants, vehicle, or roadway should be immune to undue damage. A safe level of wear and tear must be accepted.

Links:
• Circular and Transition Curves (Spirals)
• Roadway Profiles
• Geometric Design Standards
• Traffic Engineering Models
• Route Location and Alignment principles
• Route Preliminary & Location Alignment
• Route Vertical Alignment and curves
• General List of Transportation Topics
• TRANSPO Introduction start page
• Home
• Index of MYNET files

End to date, ams 990709