Topographic Surveying
The process of determining the positions of both natural and man-made features on the earth's surface and the configuration of the terrain (relief). The end product is a Topographic Map.
Stadia Method
A rapid and efficient method of surveying used to determine both horizontal distance and difference in elevation between a transit/theodolite station and a stadia rod. It utilizes the principle of similar triangles.
Interactive Stadia Method Simulator
Difference between upper and lower stadia hair readings on the rod.
Ratio of focal length (f) to stadia hair spacing (i).
Results
Formula: D = K * S + C
Calculation: D = (100)(1.50) + 0
Horizontal Distance (D) = 0.00 m
D = 0.0m
Stadia Principles
- Stadia Hairs: Two horizontal crosshairs inside the telescope used to read the intercept on the rod.
- Stadia Intercept (): The difference between the upper stadia hair reading and the lower stadia hair reading.
- Stadia Interval Factor (): Usually 100. It is the ratio of focal length to stadia hair spacing ().
- Stadia Constant (): Usually 0 for modern internal-focusing telescopes. It is the sum of the focal length and the distance from the objective lens to the center of the instrument ().
Horizontal Distance (Horizontal Sight)
Contour Lines
An imaginary line connecting points of equal elevation. The vertical distance between adjacent contour lines is called the Contour Interval.
Interactive Contour Interpolation
Determine the exact locations of contour lines between two known points.
Calculated Locations
Contour 110m:11.11m from A
Contour 115m:22.22m from A
Contour 120m:33.33m from A
Characteristics of Contours
Visual Guide
- Steep Slope: Contour lines are closely spaced.
- Gentle Slope: Contour lines are widely spaced.
- Uniform Slope: Contour lines are evenly spaced.
- Hill: Closed concentric loops with increasing elevation towards the center.
- Depression: Closed loops with hachures pointing inward, indicating decreasing elevation.
- Ridge: V-shaped contours pointing downhill (towards lower elevations).
- Valley: V-shaped contours pointing uphill (towards higher elevations).
- Overhanging Cliff: Contours merge or cross only in this case (or a cave).
- Perpendicular Crossing: Contours cross streams and ridge lines at right angles.
Interpolation of Contours
Given a grid of spot elevations, contours can be drawn by finding points of specific elevation between known points.
Methods:
Interpolation Methods
- Estimation: Visual interpolation based on experience. Quick but less accurate.
- Arithmetic Calculation: Using linear proportion. Most accurate.
- Graphical Method: Using a scale or triangle to subdivide the line segment.
Modern Topographic Representation
Modern surveying relies heavily on digital representations of topography rather than just printed contour maps.
Digital Terrain Models
- Digital Elevation Model (DEM): A continuous grid of cells (raster), where each cell contains an elevation value. Often generated from satellite imagery, LiDAR, or photogrammetry. Used heavily in GIS software.
- Triangulated Irregular Network (TIN): A vector-based representation of the terrain surface. It is constructed by connecting measured, irregularly spaced surface points (from a Total Station or GPS survey) into a network of non-overlapping triangles. It accurately models ridges, streams, and breaks in slope.
Plotting Methods
Plotting Methods
- Grid Method: Dividing the area into squares and determining elevations at corners. Commonly used for borrow pits and grading.
- Tracing Paper Method: For complex interpolation where multiple trials are needed.
- Digital Terrain Model (DTM): Using software (TIN) to generate contours automatically from point data.
Key Takeaways
- Contour Line: Connects points of equal elevation.
- Contour Interval: Vertical distance between adjacent contours.
- Slope: .
- Steep slopes have close contours; Gentle slopes have wide contours.
- V-shapes point uphill for Valleys, downhill for Ridges.
- TIN: Creates a terrain surface from non-overlapping triangles connecting measured points.