Sources of Error in Plane-Table Work
Sources of error in plane-table operation include
the following conditions or procedures:
1.
2.
3.
4.
5.
6.
Table not level
Orientation disturbed during detailing
Sights too long for accurate sketching
Poor control
Traversing and detailing simultaneously
Too few points taken for good sketching
Mistakes in Plane-Table Work
Some typical mistakes made in plane-table work
are as follows:
1. Detailing without proper control
2. Table not level
3. Orientation incorrect
DEVELOPMENT OF A
TOPOGRAPHIC MAP
In this final section on topography, we will discuss
the typical steps leading to the production of a
topographic map. In this discussion, you should notice
the different operations that are commonly involved
and how those operations interplay with one another.
In developing a topographic map, you should first
gather all available maps, plans, survey data, and
utilities data that pertain to the site and study them
carefully. Consider the boundaries of the site in
relation to the intended use of the topo map. If the map
is to be used for design purposes, certain off-site
information will be even more important than on-site
details; for example, the location and elevations of
utilities and nearby streets are vital. The location of
drainage divides above the site and details of outfall
swales and ditches below the site are necessary for the
design of the storm drainage facilities. Topographic
details of an off-site strip of land all around the
proposed limits of construction are necessary so that
grading can be designed to blend with adjacent areas.
Decide what datum and bench marks are to be used;
consider previous local surveys, U.S. Coast and
Geodetic Survey (USC&GS) monuments, sanitary
sewer inverts (not rims—they are frequently
adjusted), and assumed datum. Determine whether
there is a coordinate system in the area monumented
sufficiently for your use; if not, plan to use assumed
coordinates. In the latter case, decide on the source of
the meridian: adjacent surveys, magnetic, assumed, or
shooting the Sun or Polaris (discussed at the EA1 level
in Part 2 of this TRAMAN).
Next, perform a reconnaissance survey. Observe
the vegetation and decide how many men that you, as
party chief, will need to cut brush. Select main control
traverse stations at points appropriate for plane-table
setups. Decide on the number and location of crossties
or secondary traverse lines needed to provide
sufficient plane-table stations. Select these points so
that plane-table setups will have to be extended only
a minimum distance before checking back into
control.
The next step is to run the traverse lines; you
should check their directions from time to time, where
necessary, on long traverses. Checks could be done by
astronomical methods (Part 2 of this TRAMAN), by
cutoff lines, or by connecting the traverse with
established points. Then run the levels, taking
elevation on all traverse stations. Close, balance, and
coordinate the main traverse. Then adjust the crossties
into the main traverse. Balance the levels. Plot the
traverse stations by coordinates on the plane-table
sheets. Be sure that each sheet overlaps sufficiently.
Also, be sure there is sufficient control on each sheet
for orientation and for extension of setups (if
necessary). Number the traverse stations with the
same numbers marked on the guard stakes in the field,
and show the elevations.
The plane-table work is the final big step of the
fieldwork, but some transit and level work may still
need to be done. The location of some details (such as
street center lines or buildings) may need to be more
precise than the precision obtainable with the plane
table; tie in such details to the traverse by transit tape
survey. For design purposes, the elevation of some
points (such as the inverts of culverts, paved flumes,
sewers, and tops of curbs and gutters) may need to be
more precise than the precision obtainable with the
plane table. Use the level to obtain such elevations.
The final step in the production of the topographic
map is, of course, tracing the information from the
plane-table sheets onto the final drawing.
Random traversing, as previously described, is not
the only way of establishing horizontal control. Grids
are frequently used. One good way of identifying grid
lines is to assign a letter to each line in one set and then
run stationing along each line. Another method is
described in the paragraphs below.
9-9
