Topographic surveys and maps

Topographic surveys find the location of existing features on the land. Map as a result provides a description of smaller area with more details involved compared to other maps.

Cultural features and important objects

Example of symbols which can be shown on a topographic map

Information about surface relief

Example of symbols which can be shown on topographic map

Natural features

The result of topographic survey (also sometimes called preliminary survey) is plotted on the topographic map (or topo map). Topo maps have a wide range of usage:

Contour lines

Topo maps are useful in showing the surface relief to its user. The common way to show the terrain or give the impression of a third dimension is to use contour lines. Contours join equal elevation.

1—Model, 2—Topographic map as the result from model
Contour lines trace the elevation of the terrain surface at regularly-spaced intervals.
Drawing contour lines is a way to represent a terrain surface with a sample of elevations. The more imaginary planes, the more contours, and the more details are captured.

The contour lines depict height over sea level and are drawn in contour lines intervals. The interval is a vertical distance, which is chosen according to terrain and according to the scale of the map. So, convenient reading of contour lines is provided. The interval might be for example every 10 m, every 50 m.

Topo map showing contours
Engineers, architects and building contractors base their construction projects on maps from topographic or land surveys to help design buildings, dams, housing projects, canals, drainage pipes and other structures.
Topographic maps from surveys would indicate the best area for building the structures, how much dirt to move, and the possible obstacles the project could encounter.
Topographic survey for construction project: projects that are designed to build or improve infrastructure including building facilities, parks/open space, streets, sidewalks and sewers need planning

How the contours can be measured

  1. First, the area being surveyed is considered in general
    to highlight locations with a greater need for more descriptive survey. For example, a flat area against area in a steep slope (the second one will be subject to more detailed survey)
    .
  2. In order to prepare a topo map, it is needed to establish control points in surveyed area.
    Usually by traversing. Location of the control points will be chosen in accordance with the previous point.

  3. According to demanded descriptiveness heights of

    1. only the control points are further measured or
    2. control points and rod shots nearby are measured or
    3. grid of grid points is measured
      (establishing the grid does not require a high degree of accuracy)
      .
    4. Combination of the grid and rod shots can be chosen.
  4. By means of differential leveling, the heights of either control points or grid points are found in the field survey.
  5. It is not expected that the measured heights lie exactly on the contour.
    Thus, interpolation is used to find contour lines locations between points. The interpolation assumes that the slope between points is uniform (linear interpolation).
  6. There are many ways of doing interpolation. The result is always only approximate and often interpolation by eye is used.
Using interpolation to find the location of contour line of 95 m altitude
  
  
1—In the grid contour lines connecting multiples of 10 are drawn.
2—More complicated example, hills and plains can be seen (contour lines at altitudes of 10, 15, 20, ...).
There is no need to be restricted by equidistant grid only. Grids are chosen and located according to the conditions of the area.

Survey procedures

There are several ways how data can be collected in field for determining the positions of points. On the picture below are depicted most straight-forward methods for locating objects.
The baseline is derived from traverse..
Common methods for determining position from baseline:
1—Baseline of known location and object to measure
2—Polar coordinates (angle and distance) are taken
3—Rectangular coordinates (plus and offset) are taken
4—Linear coordinates (two distances) are taken
5—Focal coordinates (triangulation) are taken
6—When position is determined, sides of the building are taken also

In general it is always recommended using roughly equilateral triangles to achieve good accuracy.

Illustration of, why we should use roughly equilateral triangles in surveys.
Let's assume that we take triangle by lengths of its sides (very similar conclusions will be found if instead of sides angles are taken):
(top)—The real and expected shape of triangle
(middle)—The error was involved during a survey (for all shapes the error is roughly of the same size)
(bottom)—The result triangle involves the error from field. On wide and sharp triangle the error is multiplied and reconstructed triangle does not remind expected shape much.

Estimating the right angle in the field (perpendicular direction)

Oftentimes it is not needed to find the right angle accurately.
Three methods to establish estimation of right angle are listed:

Right angle using arms

Surveyor stands with his arms wide over baseline AB. Then he moves arms forward to the target and compares if his hands point to the target. If hands do not point to the target, he moves himself to the proper side and repeats the steps again and again until the position is found. This is a trial-and-error method.

Trial-and-error method to find foot of the right angle

Right angle using double pentaprism

Double pentaprism is a tool for surveyor which consists of mirrors reflecting the incoming beam at an angle of 90 ° (regardless its position against the source of the beam). If two pictures observed through pentaprism are aligned then right angle is found.

1—Pentaprism is a set of mirrors which deviate a beam under 90 °. The beam reflects inside prism twice.
2—Penta prism—a tool for surveyor
3—The corner of the building is observed against rod to check the foot of right angle

Right angle using swinging a tape against theodolite sight

Theodolite is established in line with the baseline. Tape is hold from the target, that means perpendicular to the baseline. Tape is being swung (rotated) around supposed right angle and reading is observed from theodolite. The right angle is found when the theodolite crosshair points to the shortest value on the tape during swinging.

Theodolite is established in line with baseline AB and sight is being observed. A rod or a tape is placed from target and is being waived/swung. When shortest reading on tape/rod is observed from the rod, then the right angle foot position is found.

Basic photogrammetry

It might be needed to make topographic map of large area.
If the area is around 1 km2 (and more), it is usually efficient to use photogrammetry
instead of classic surveying methods
.
Photogrammetry involves precise measurements on photographs taken by camera from an aircraft to determine relative distances of objects. Distances and elevations can be accurately measured.
Contour intervals smaller than 1 m can be plotted.

General purpose topo maps are useful to determine

Basic principles

The field work is considerably reduced but not eliminated.
Classic surveying methods will be used to locate some control points. The accuracy of map depends on accuracy of ground survey. The points must be clearly identifiable on the photos. Either existing control points can be used (eg. crossroad, corner of buildings), or artificial control points are being placed prior obtaining aerial photos.
The lap of surrounding pictures is about 60 %. The overlap allows to use stereovision and stereoscopic plotting to create contour lines.

Photos are taken with the camera from aircraft. The axis is aligned with direction of gravity. Aerial photographs are taken along a series of overlapping paths. One object appears on 4 or more pictures. The lap of surrounding pictures is about 60 %. The overlap allows to use stereovision and stereoscopic plotting to create contour lines.

Relief displacement

If the tall object (eg. tower, chimney, skyscraper) is on the vertical axis with the camera, it will be observed like a point. The observation will be different if the same object is far from the center: it will be printed as a line. The amount of displacement can be measured by micrometric rule and using geometry its the height can be calculated.

1—A ground orthophoto and 2—a true orthophoto
3—Relief displacement on scheme
Although relief displacement is not welcome when photo is used as a map (the building footing hides the streets and other objects) thus have to be avoided, it provides useful information about the height of the objects when processed properly.
Relief displacement
is a topic not only when considering tall objects as above. It
must be evaluated when there are hills, valleys and other relief involved.
Their location must be fixed, because these are not being projected into datum ("zero" altitude) on photograph.

Stereovision

Either human's or animal's eye is the organ to receive visual information. Human can use a pair of eyes for binocular vision.

The field of view observed from the left eye overlaps with the field observed by the right eye. Because the left and right eye position slightly differs in observing position, these two images differs slightly as well (differs more if observed object is close to the eyes).

The brain permanently evaluates left and right pictures into one picture with accurate perception of depth.



There are two examples of stereovision. Each pair consists of a picture for the left eye and a picture for the right eye. You can use your eye muscles to overlap both pictures into one. It is similar as if you are looking at something far behind the display. Easier to achieve if display/picture is not too close to eyes. Then you will experience 3D illusion right now. Warning: your eyes can became tired soon.

Stereoscope is a device to provide one photograph only for the left eye and other photograph for the right eye. Using this device to watch photographs, the illusion of 3D perception can be achieved by the observer. The photographs are taken from different positions from aircraft and are prepared to overlap. The same principle is used for 3D cinemas.

Stereoscopic plotting of topo maps

The principle of stereovision is used to prepare topographic maps, which show the shape of the ground.

1—Galileo Santoni III Stereoscope from 60's and 2—Zeiss Planicomp P3 Stereoscope from today
Instruments called stereoplotters make it possible for operator both to observe and analyze 3D image
of the ground by viewing overlapping aerial photographs. He can navigate microdot over the picture to plot ground elevation contour lines on a map sheet
.

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