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