Mapping Terminology: Topographic Maps and Control Points

Mapping Terminology

• Topographic maps are detailed illustrations of man-made and natural ground features like roads, railways, power lines, contours, elevations, rivers, lakes, and geographical names.
• Control points are reference points with known coordinates that establish a consistent coordinate system for construction projects.
• Land surveyors use control points to accurately position elements on a jobsite, using digital plans or Building Information Modeling (BIM).
• Contour lines are imaginary lines on Earth's surface connecting points of equal height above sea level that illustrate the shape or relief of an area.
• Contour interval refers to the constant difference in height between contour lines, which simplifies height calculation.
• To calculate height use the contour interval, count the lines, and multiply by the interval.
• In a map with elevations 20 and 30, with 2 contour lines in between, the contour interval is 5.
• Contour maps use contour lines to depict valleys, hills, and the steepness or gentleness of slopes.
• Relief maps show the elevation, high points, and depressions, low points of a given area.
• Strip maps present a linear representation of a specific route for navigation or travel.
• Open traverse refers to a sequence of connected lines extending along a general direction without returning to the starting point.
• Closed traverse is a series of connected lines with measured lengths and bearings that enclose an area.

Surveying Methods

• Compass surveys determine the directions of surveying lines using a magnetic compass.
• Lengths of surveying lines are measured with a tape, chain, or laser range finder; compasses are used to run traverse lines.
• The Compass rule, also known as the Bowditch Rule, applies to traverses with similar angular and linear accuracy.
• The transit rule assumes course directions are measured with higher precision than distances.
• Stadia method is a surveying method for determining distances and elevation differences using a telescopic instrument with two horizontal lines.
• Optical readings are taken through a transit or theodolite.
• In stadia principle, the relationship between horizontal distances (H) and rod intercepts (V) is based on Similar Triangles.
• Triangulation is a surveying method that measures angles in a triangle formed by three survey control points.
• Trilateration is a surveying method where the lengths of a triangle's sides are measured, usually electronically, to compute angles.
• Trilateration estimates a mobile's position by measuring distances from reference points, also known as range measurement techniques.
• Triangulation locates a mobile by computing angles relative to different reference points.
• Trilateration and triangulation are considered more economical than the stadia method.
• Right-angle offset method determines the location of points using perpendicular distances from a survey baseline.

Other Terminology

• Electric currents in Earth's molten iron core are the primary source of Earth's magnetic field.
• The direction of the horizontal component of Earth's magnetic field defines the magnetic meridian.
• Grid meridians provide a consistent north-south reference aligned with a map's coordinate system in navigational systems.
• When magnetic declination is 0 degrees, magnetic and true meridians align, eliminating the need for compass corrections.
• The two-angle method locates a point by measuring two angles from two known control points.
• This method is suitable when the object is inaccessible, like across a river or highway.
• The Two distances method locates a point by measuring two distances from two known, fixed points.
• Best used when an outbuilding is obscured, preventing direct traverse measurements.
• Angle and distance method (azimuth-stadia) determines a point's position by measuring both the angle and the distance from a known point.
• Stadia two distances method, a stadia tacheometry technique, uses a telescope with stadia hairs to determine distances.
• Cross-sectioning determines the shape and elevation of the ground perpendicular to a central line and its essential for linear construction projects.
• Grid systems are most appropriate for determining elevations at regular intervals when creating area maps.