The acquisition of data regarding such details must be as 
comprehensive and detailed as essential for completion of the planning 
process. Absolute accuracy in relation to fixed datum references need 
not be achieved. The relative difference with respect to identifiable 
features, both on and off the highway, of course, becomes essential. 
The speed of vehicles, for example, in miles per hour, or the distance 
between vehicles need not be referred to a particular datum. Accord- 
ingly, precision in an absolute sense is seldom, if ever, required. 
Ordinarily, speed in miles or kilometers per hour, or distance between 
vehicles in feet or meters, is adequate. To do measuring by photo- 
grammetric methods, which will make it possible to determine speed 
and/or spacing, reference can be made to naturally existing features, 
or to targets especially set on the shoulders, or on the pavement. 
The distance between such features or the special targets on the 
shoulders and the pavement need be determined on the ground only to 
the nearest foot or meter, without regard for fractions. 
The maps compiled and photographic mosaics prepared and utilized, 
and augmented by stereoscopic examination of aerial photographs, can be 
small in scale. The accuracy of these maps, of course, would be com- 
mensurate with their scale, but need not exceed what is feasibly 
achievable by the elemental photogrammetric use of aerial photographs 
controlled by use of supplemental control survey measurements made on 
the ground or by photogrammetric methods to satisfy what is sometimes 
referred to as fourth order requirements. Fourth order surveys require 
an accuracy of 1:2,500 for horizontal measurements and for the vertical 
measurements of 0.50 of a foot times the square root of the length of 
the level circuit in miles or 0.12 of a meter times the square root of 
the length of the level circuit in kilometers. 
Determination of Feasible Routes. 
Once the planning has been completed, including designation of 
termini and establishment of priority for work accomplishments, it is 
essential that the most feasible routes be determined between the 
termini. In this stage, absolute accuracy is not a fundamental 
essentiality. Relative distances, however, are important. By relative 
distances are meant the height of a pass above a valley, and the 
distance required to design and construct a highway on a suitable 
gradient from the pass to the valley floor. Usually the elevation is 
measured to the nearest ten feet, or three meters, and the horizontal 
distance is measured to the nearest one hundred feet, or 30 meters. 
There are exceptions, of course, where distance measurements to the 
nearest meter, or about three feet, are desirable. Nevertheless, an 
absolute distance referred to a finite datum in either case is some- 
what unnecessary, although it will be beneficial for achieving corre- 
lation throughout the subsequent engineering stages. This is so 
because determining the feasibility of a highway route between desig- 
nated termini is dependent upon many significant aspects which are not 
affected by absoluteness of measurements, such as, for example, the 
type of soil; ground conditions, tendency for or lack of ground sta- 
bility; type, prevalence, and magnitude of drainage patterns; the 
economic effects of the highway on value of land which vill be 
traversed; the availability of suitable construction materials; the 
rate of horizontal curvature and gradients permissible and required 
&ccording to the design speed for the proposed highway; the cost of