Immediately after the alignment of a line has been finalized to the satisfaction of both the engineer and the borrower, a survey should be made to map the route of the line. Based on this survey, plan-profile drawings will be produced and used to spot structures.
Long corridors can usually be mapped by photogrammetry at less cost than equivalent ground surveys. The photographs will also contain information and details which could not otherwise be discovered or recorded.
Aerial survey of the corridor can be accomplished rapidly, but proper conditions for photography occur only on a comparatively few days during the year. In certain areas, photogrammetry is impossible. It cannot be used where high conifers conceal the ground or in areas such as grass-covered plains that contain no discernible objects.
Necessary delays and overhead costs inherent in air mapping usually prevent their use for short lines. When using photogrammetry to develop plan-profile drawings, proper horizontal and vertical controls should first be established in accordance with accepted surveying methods.
From a series of overlapping aerial photographs, a plan of the transmission line route can be made. The plan may be in the form of an orthophoto or it may be a planimetric map. The overlapping photos also enable the development of profile drawings.
The tolerance of plotted ground elevations to the actual ground profile will depend on photogrammetric equipment, flying height, and accuracy of control points.
Survey data can be gathered using a helicopter-mounted laser to scan existing lines and/or topography. Three dimensional coordinates of millions of points can be gathered while also taking forward and downward looking videos. These points can be classified into ground points, structure points and wire points.
If use of photogrammetry or laser-derived survey information for topographic mapping is not applicable for a particular line, then transit and tape or various electronic instruments for measuring distance should be used to make the route survey. This survey will generally consist of placing stakes at 100 foot intervals with the station measurement suitably marked on the stakes.
It will also include the placement of intermediate stakes to note the station at property lines and reference points as required. The stakes should be aligned by transit between the hub stakes set on the preliminary survey.
The survey party needs to keep notes showing property lines and topographic features of obstructions that would influence structure spotting. To facilitate the location of the route by others, colored ribbon or strips of cloth should be attached at all fence crossings and to trees at regular intervals along the route (wherever possible).
As soon as the horizontal control survey is sufficiently advanced, a level party should start taking ground elevations along the center line of the survey. Levels should be taken at every 100 foot stations and at all intermediate points where breaks in the ground contour appear.
Wherever the ground slopes more than 10 percent across the line of survey, side shots should be taken for a distance of at least 10 feet beyond the outside conductor's normal position. These elevations to the right and left of the center line should be plotted as broken lines.