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