Alidade Mapping

Plane Table Mapping

A plane table consists of a drawing board mounted on a tripod with a leveling device designed as part of the board and tripod. The commonly used leveling head is the ball-and-socket type. The cross section of a plane table with the tripod head is shown in figure 11-30. The board (G) usually is either 18 by 24 in. or 24 by 31 in. and has an attached recessed fitting that screws onto the top of the spindle (A). A wingnut (B) controls the grip of parts C and D on cup E. By releasing the wingnut (B), you can tilt the drawing board in any direction to level it. Another wingnut (F) acts only on the spindle and, when released, permits the leveled board to be rotated on azimuth for orientation. The tripod is shorter than the transit or level tripods and, when set up, brings the plane table about waist high for easy plotting. One precaution must be observed in attaching the plane table to the tripod head.

Mapping with a Plane Table and Alidade

First position of the plane table

* attach a blank sheet of paper to the plane table

* choose an appropriate scale according to the relative sizes of the mapping sheet and the study area

* mark two base points (A' and B') on the paper corresponding to two ground positions (A and B) of known separation and relative location

* position the plane table at A, placing the alidade along the line A'B' and turning the table with attached map until the A'B' line-of-sight coincides exactly with its corresponding AB line

* without moving the plane table (even a slight shift in map orientation will cause errors), move the alidade so another field position, say C, can be sighted from A

* a penciled line along the edge of the alidade will form a ray from A' toward a position that will become C'

* in a similar manner, rays can be drawn toward other visible field objects

Second position of the plane table

* position the plane table at B

* place the alidade along line B'A' and turn the table and map sheet so the B'A' line matches the BA line-of-sight

* if the B'A' line coincides exactly with the BA sighting line, then the map is oriented the same as it was at position A

* now, rays drawn from B' toward C and D will intersect those drawn from A'

* the intersections determine points C' and D' in the manner that a side and two adjacent angles always geometrically determine a triangle, i.e., by triangulation.

Subsequent positions of the plane table

* the researcher can move the plane table to position C because its corresponding map location is known, and therefore can repeat thc triangulations and extend the map data

* moving to a third field position, whether or not it is necessary to view all targets, enables the mapper to verify the accuracy of the previous work

* if the rays from three positions do not intersect at a single point, then the triangle of intersections represents the degree of inaccuracy of the map

* three rays may not intersect at a common point if two, or all three, are almost parallel; thus rays that intersect at a very small angle generally should be avoided

Geomorphological mapping

* recognition of basic landscape units that are easily identified in the field, on aerial photographs or from maps

* the basic unit is scale-dependent; a landform is a single unit at one scale and an assemblage of units at a larger map scale

* thus scale and objective (e.g., hazard mapping, surficial deposits/ aggregate resources, terrain analysis) determine the content of a geomorphic map

* some maps are entirely morphological (descriptive), but most include an interpretation of the origin and / or age of landforms or deposits

* detailed geomorphic maps tend be misleading in their precision which hides the subjectivity behind mapping

* mapping is an activity that can be of greater benefit to the mapper than the user by forcing the mapper to recognize and contemplate the complexity of a landscape (sketches serve a similar purpose)

* slope profile surveying is a similar activity that provides great appreciation for the subtleties of hillslope form profiles are useful in a variety of contexts and thus there are manuals and preferred methods