Geomatic Engineering Notes

Elevations and Chainage
  • Various elevations observed:
    • -3.845
    • -4.550
    • -4.765
    • -4.805
    • -4.965
    • -5.150
  • Chainage measurements include 0, 10, 20, 30, 40, 50, 60, and 70.
Underground Levelling (Inverted Sights)
  • The leveling staff is positioned upside down, with its foot on the measurement point.
  • In data recording, inverted staff reading is recorded as a negative value.
    • Example: An inverted reading of –3.456.
  • Rise and Fall Method Calculation:
    • To find elevation difference from point B (elevation 2.248) to point A:
    • Calculation:
      • extDifference=2.248−(−3.456)=2.248+3.456=+5.704ext{Difference} = 2.248 - (-3.456) = 2.248 + 3.456 = +5.704
    • This value can be added to B's elevation to find A's elevation.
    • The reverse calculation from A to B:
    • extDifference=−3.456−2.248=−5.704ext{Difference} = -3.456 - 2.248 = -5.704
    • Add this to A's elevation to obtain B's elevation.
  • Dealing with inverted readings is simplified by recording them as negatives and employing standard algebra.
Contouring and Cross-Sections
  • Contour Definition:
    • A contour line connects all points at the same elevation relative to a datum.
    • Contours cannot cross except in special conditions (e.g., overhanging cliffs).
  • Contour Interval:
    • The vertical spacing between successively higher or lower contours. This is vital for engineering projects requiring detailed knowledge of topography.
  • Spacing and slope representation:
    • Closely spaced lines indicate steep slopes; widely spaced lines indicate gentle slopes.
  • Contour Maps:
    • A map with contour lines indicating ground features and elevations, serving both plan and profile purposes.
  • Closed contour lines indicate depressions if higher values are outside, and hills if higher values are inside.
Contouring Methods
  • Types:
    1. Direct Method: Required for high accuracy, identifying ground points for contours directly.
    2. Indirect Method:
    • From Random Spot Heights: Collect height data at random intervals; suitable for small hills.
    • From Grid of Spot Heights: Establish a grid and obtain levels at intersections.
Profile and Cross-sectional Levelling
  • Objective: Determine elevations of points to create a vertical profile along a specified line, useful for roads and pipelines.
    • Profiles are plotted on a larger vertical scale compared to the horizontal scale.
  • Cross-sectioning:
    • Provides a short profile perpendicular to a project's route (e.g., highway). Useful for estimating earthwork volumes and slope grades.
    • Cross-sections are taken at specified intervals.
Contour Interpolation
  • The procedure for spacing contours proportionally between plotted level points.
  • Methods:
    1. Mathematical Calculation: Using proportions based on spot heights to find contour positions.
    • Example calculation detailed:
      • Given two spot heights, calculate point position using proportions.
    1. Graphical Method: Drawing contour lines based on graphical representation of height differences.
    2. Estimation Method: Rough estimation of contour points between identified ground points.
Uses of Contour Maps
  • Provides information on ground features (flat, undulating, mountainous).
  • Aids in visualizing intervisibility between points and enables layout planning for engineering projects.
  • Offers insights into earthwork quantities needed for construction.
Profile and Cross-Section Measurement
  • Record distances, back sights (BS), intermediate sights (IS), and foresights (FS) to determine heights and reduced levels.
  • Crucial for planning and executing projects accurately.