SITE PLANS AND PROPILES. Geological data affecting foundation designs at construction sites are plotted on plans drawn to scales of 1 inch = 50, 100, 200, or 400 feet. Contour intervals may range from 1 to 10 feet, depending upon the roughness of the terrain Plane table mapping is suited to plotting the topographic features, ranges, and reference points used to locate drill holes, rock outcrops, and other geologic data. When plotting contours on a 1- or 2-foot interval, you should try to locate points that are actually on the contours or to determine elevations at the intersection of closely spaced grid lines staked out on the site. In addition to a plan, the geologist may require that profiles be drawn along selected lines or that the boring logs of test holes be plotted to suitable scales.

USING A TOPOGRAPHIC MAP AS A BASE MAP. The base map for a detailed geological survey is a complete topographic map or plan with relief expressed by contours. Simple colors and symbolization of basic details are used so that they will not conflict with the overlay of geological information that is shown by colors and symbols. Published topographic maps are used where suitable. The geological survey is expedited if the map base is from a quarter to double the scale of the map on which the information is to be presented. Enlargements of the base map, rather than other maps of a larger scale, are generally used to satisfy these requirements. This permits the direct reduction of geological data to the scale of the final map with a minimum amount of drafting.

Sometimes there is a requirement for pedological mapping for the purpose of locating the limits of sand or gravel deposits suitable for concrete aggregates, road materials, or for other construction operations. In such a case, the pedological survey conducted under the direction of the soils engineer and the surveyors mission would be one of support to the soils engineers objective.

The engineers objective in a pedological survey is to prepare data in plan and profile symbolizing soils and outcropping on maps, overlays, and sketches for subsequent engineering uses. The following approaches may be used in conjunction with a soils survey operation:

1. Aerial photography may be used when an extensive area is to be surveyed. Usually no survey measurements are required in this case.

2. Maps of an area that extend several square miles are required when an initial study or technical reconnaissance is needed for an engineering project. Low-order survey measurements usually suffice for the preparation of a reconnaissance sketch upon which the soils engineer can plot the pertinent data.

3. A sketch of an airfield, for example, is frequently required by the soils analyst before construction planning can be initiated. In this case, the surveyor applies low-order measurements to prepare a sketch (1 inch = 100, 200, or 400 feet) upon which the soils engineer plots the results of soil tests and findings.

Photo coverage of the area under consideration aids in the establishment of control for the pedological survey. The use of vertical aerial photographs in the planning phase of outlining ground control will speed the survey regardless of the size of the area to be covered If controlled photographs are available, the survey engineer can locate points by pricking or keying them to the photographs. An uncontrolled photograph may be satisfactory for the surveys of low-order accuracy mentioned in the preceding paragraph.

The plane table traverse is best adapted to relatively open country for the preparation of the basic sketch upon which the soils engineer plots pertinent data. In the absence of detailed instructions from the soils engineer, the following procedures are generally satisfactory for preparing a sketch of an area of several square miles (3 miles by 3 miles maximum for initial exploration):

2. TRAVERSE CONTROL. Run in circuits or between known positions of a higher order of accuracy.

3. SIGHTING. Use a peep sight or telescopic alidade.

6. COMPASS. Use military compass, forestry compass, or pocket transit.

7. DISTANCE BETWEEN BASIC CONTROL POINTS. Maintain 3 miles as the extreme maximum distance between stations.

8. ACCURACY. Distances should be measured in such a manner that points can be plotted within 25 feet. For the scales suggested, measurements to 1 part in 100 will suffice. Take sights with peep-sight alidade carefully to maintain directions of an accuracy comparable to distances.

9. TOPOGRAPHY. Topography is usually not required on reconnaissance surveys for pedology, particularly in areas of low relief. Where suitable deposits of sand, gravel, or stone have been located route surveys from the site to the point of use may be required for the location of haulage roads, conveyors, or other means of transporting the material. In hilly terrain, a rough topographic map, obtained by clinometer, pocket transit, or stadia, may be required to make the location of a favorable route easier.

 In heavily wooded areas, compass traversing is more convenient than plane table traversing; however, more time is required for plotting by the compass traverse method. Traverse lines between stations should be long to reduce the number of observed bearings. Points between stations are located by offsets from the traverse lines. Where local attraction affects compass readings, points are plotted by intersection. Survey readings may be plotted in the field. Notes should be kept in case the traverse must be retraced. In the absence of detailed instructions from the soils engineer, the basic guides for plane table traverse apply.