Field Surveys for Mapping

Offsets from a Reference Line

Collection of data for mapping is based on making sufficient measurements to allow mapping of the feature in an office. One of the simplest methods of producing such maps uses nothing more than a tape to collect field data. This procedure has been often used in alignment surveys where only objects on the side of the alignment are needed. In this procedure a baseline is established along the alignment. From this baseline perpendicular offsets are measured to the needed features. This is shown in Figure 1 where a is an offset distance measured from a baseline established along the edge of Oak Street. These surveys are useful for mapping building and vegetation offsets from a street on which construction to be performed. The field book for such a survey could be written as

Table 1 Field notes for alignment survey.

Feature	Stationing	Offset	Description
House1	0+13.7	33.3 (a)	South corner of house
House 1	0+43.5	33.5	North corner of house
Oak1	0+48.2	25.8	17" Oak tree
House2	0+89.5	54.7	South corner of house
Birch1	0+99.1	45.1	6" Birch tree

Radial Surveys

With the advent of the EDM, and more recently the total station, it has become increasingly popular to perform radial surveys. This method of surveying is extremely efficient with total stations which simultaneously measure angles and distances to a feature, and are most efficient when the instrument is placed in a location that can see large areas to be mapped. Figure 2 show a radial survey of a property with two outbuildings and a house. Note that both a backsight station and an instrument station have been established. Often these stations are part of a larger control traverse, and are oriented to some common coordinate datum such as the State Plane Coordinate System.

While a theodolite-EDM combination with a written field book can be used for these surveys, it is more efficient to use a total station with an electronic data collector. In combination with field-to-finish software, the final planimetric or topographic map can be completed quickly upon return from the field. However for this to occur, field personnel must understand the capabilities and limitations of this software.

While no two data collectors are exactly the same, most contain many similar features. For instance if a tree is to mapped the problem of where to place the reflector occurs. If the reflector is placed on the front surface of the tree as shown in Figure 3a, the distance to the tree, and thus its plotted position will be offset by the radius of the trunk. If the reflector is placed to the side of the tree as shown in Figure 3b, the distance to the tree will be correct, but the angle to the tree will be wrong. Thus data collectors have features that allow users to collect a separate distance and angles to the trees and other features with similar problems. This capability allows the user to measure the angle and distance to the center of the trunk as illustrated in Figure 3c. Other common features include the ability to include store and recall notes. This feature is important since the field-to-finish software often requires certain terminology for correct plotting of features. For instance to place the map symbol for a deciduous tree at its measured position, the software may require the note field start with DTREE. Each software package is accompanied by a symbol library that matches field codes with graphical symbols. Thus field crews must be aware of the capabilities, restrictions and limitations of the software and use proper field coding to ensure correct graphical representation after downloading.

Line Work

Most field-to-finish software has the capability of taking line work collected in the field and upon reduction graphically displaying this work. Again, it is important for the field crews to understand this capability and the software limitations that accompany this capability. For instance the house shown in Figure 4, may have notes that are required to appear as

In this table, we see that a period is used by the software to indicate the start of a line at 1, this nomenclature is continued until the last measurement at 4 which is followed with a plus (+) sign. The plus sign indicates that the line is to be closed back onto the initial point. It is important to realize that the order of the shots taken in the field is extremely important. For instance, if the shots had been taken in the order of 1 - 2 - 4 - 3, the subsequent line work would be incorrect with the lines drawn from 1 to 2 to 4 to 3 to 1. Table 2 shows the line drawing designators that can be used with the Civil Series software from Eagle Point.

Table 2 Line work designators from Eagle Point.

For more on the meaning of these symbols and other items on linework goto the Data Collection Manual from EaglePoint on pages 37 - 65. A summary of the table is availble by selecting this link.

Using the codes listed in Table 2, the notes for the alignment would be

Note that feature 7 is followed by a hyphen to indicate that this point is on a curve. When the software reduces this field work, the it will automatically draw a line called CBR starting at 5 turning through 6, 7 and 8 and stopping at 9. When multiple features intersect at one survey point, the notes must be handled in a special format. An example of the Eagle Point line work is shown in Table 3.

Table 3 Examples from the Eagle Point software manual.

Data Collection for Topographic Work

Data collection for topographic maps proceeds in much the same manner as discussed above. However when working with field-to-finish software, certain field procedures must be followed. As an example, consider the profile of the ground surface shown in Figure 5. At each numbered point, either slight or distinct breaks-in-grade occur. A break-in-grade is a location where the slope on one side of the point is different from the slope on the other side of the point. To properly contour this work, break lines must be collected in the field. Break lines are lines which follow the break-in-grade and from its beginning to its end. By collecting these lines, the software will properly TIN the data and subsequently contour it correctly. Besides break lines, the field crew must also collect sufficient points between the lines to properly construct the TIN.

Triangulated Irregular Network (TIN)

TIN is an acronym for a triangulated irregular network, and is the most commonly used method for automated contouring of data. An example of a TIN is shown in Figure 6. In this figure, we see that triangles are constructed connecting each data point from the field. The dashed lines represent break lines as they were located on the ground. When the field crew is collecting grade points for contouring, the subsequent triangles should approximately form equilateral triangles. While the location of the location of planimetric field data will somewhat control the proximity of the triangles to their equilateral shape, obvious sliver triangles should be avoided at all cost. Sliver triangles are triangles that have one extremely small angle. Invariably these triangles will contour incorrectly resulting in wavy contours. The triangles shown on the right of Figure 6 are examples of sliver triangles that should be avoided.

One of the method to avoid sliver triangles and to adequately cover an area is to develop a grid for the area to be contoured. In this procedure, an approximate grid is constructed on the ground after the planimetric features and break lines have been constructed.. The size of the grid is dependent on the intended accuracy of the final map and the distance between points on the break lines. For instance, if points are gathered at an interval of approximately every 50 paces on the ground along the break line, then the most appropriate grid layout would be 100 paces. A common error is to assume that since the slope between the break lines is constant, it will contour correctly. The construction of sliver triangles is shown in Figure 7 occur when too few intermediate grade points are located between break lines. The blue line represents the undulation of the contours that occurs due to inadequate data collection. This situation could have been avoided if grade points were measured between the upper and lower break lines at intervals that approximated the data collection along the break lines.

Global Positioning System (GPS) Measurements

GPS technology is also capable of collecting data for mapping. This technology involves the positioning of a receiver in the field. Currently GPS capabilities limit the accuracy of field measured data for mapping from a few centimeters to 5 or more meters. While this technology is inadequate for the most precise mapping applications, it is capable of rapidly and efficiently collecting field data for many mapping applications. For example, if the intended final map scale is 1:1200, and the plotting accuracy is ±0.1 mm, the resulting plotting error will be 120 mm! Using kinematic surveying techniques, the user can expect accuracies from the GPS equipment on the order of 20 - 50 mm. Since this tool can collect field data at the pace of the user, and since it requires only one field person to perform this collection. It can be economical, fast, and accurate enough for many mapping situations. GPS equipment allows the user to collect data by time interval, distance, or at the user command. There are limitations to this technology due to the requirement that a GPS receiver must have visibility to the sky from an angle of approximately 15° above the horizon. However manufacturers are currently developing data collectors that work with both the GPS receiver and a total station. With this combination, it will be possible for the surveyor to map large open areas quickly with GPS and fill the gaps caused by visibility with their total station. This will result in reducing the time and effort needed to develop an accurate map.

GROUP ACTIVITY: Use your assigned total station and SMI data collector to collect sufficient data to map a room in this building with proper notes for automated drawing. (Activity points: 1.5)

GROUP ACTIVITY: Download the data collected in the previous activity, and create a map of the room using EaglePoint. (Activity points: 1.5)

GROUP ACTIVITY: Use the TURBO G1 GPS receivers to create a planimetric map of the campus at a scale of 1:2400 on A-size paper with a 1" border. (Activity points: 3)

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Contacting the Instructor.

Created by: Charles D. Ghilani
Copyright © 1999, Penn State Surveying Program