Introduction to Photogrammetry and GIS

The science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant imagery.

Aerial Photography

Photographs taken from an aircraft, drone, or satellite.

Types of Aerial Photos

  • Vertical Photography: Camera axis is vertical or nearly vertical (tilt <3∘< 3^\circ). Essential for accurate topographic mapping and scale derivation.
  • Oblique Photography: Camera axis is intentionally tilted.
    • High Oblique: Includes the horizon. Used for general views and illustration.
    • Low Oblique: Does not include the horizon. Used to see the sides of buildings or objects.

Stereoscopic Coverage

For 3D mapping and topographic extraction, aerial photographs must overlap to provide two different views of the same ground area (stereoscopy), allowing the viewer or software to perceive depth (the parallax effect).

Image Overlap

  • Endlap (Forward Overlap): Overlap between successive photos along the flight line. Typically required to be at least 60% to ensure stereoscopic coverage of the entire area and to link stereomodels.
  • Sidelap (Lateral Overlap): Overlap between adjacent flight strips. Typically required to be at least 30% to ensure no gaps exist between strips when flying a grid pattern.

Scale of a Vertical Photograph (SS)

Scale is the ratio of a distance on the photo to the corresponding distance on the ground. Unlike a map, the scale of a photograph is variable due to terrain relief.

Scale Formula

S=fH−h S = \frac{f}{H - h}
Where:

Checklist

Relationship:

Checklist

Interactive Scale Simulator

Explore how Focal Length, Flying Height, and Ground Elevation affect the photo scale and ground coverage.

Aerial Photogrammetry Scale Simulator

Photo Scale:Undefined
Ground Coverage:0.0 m

Assuming standard 23cm x 23cm format.

Ground (h)
Datum (H)
Coverage: 0m

Relief Displacement (dd)

The shift in the position of an image on a photograph caused by the relief (elevation difference) of the object. Objects above the datum are displaced radially outward from the principal point (center).

Relief Displacement Formula

d=rhH d = \frac{r h}{H}
Where:

Checklist

Use: Can be used to calculate the height of objects (e.g., buildings, towers) from a single vertical photo by measuring the displacement (dd) and the radial distance (rr) to the top.

Modern Remote Sensing

UAV and LiDAR

  • Drone Surveying (UAV/UAS): Unmanned Aerial Vehicles have revolutionized small-area photogrammetry. Using principles of Structure from Motion (SfM), software analyzes thousands of overlapping high-resolution photos to automatically generate dense 3D point clouds, orthomosaics, and Digital Surface Models (DSM). They are highly efficient for volumetric calculations, construction monitoring, and rapid topographic mapping.
  • LiDAR (Light Detection and Ranging): An active remote sensing technology that uses rapidly pulsing laser beams to measure distances to the Earth's surface. Crucially, LiDAR pulses can penetrate through gaps in forest canopies, allowing surveyors to map the bare-earth ground (Digital Elevation Model) underneath dense vegetation, which is impossible with standard photogrammetry.

Geographic Information Systems (GIS)

A computer-based system designed to capture, store, manipulate, analyze, manage, and present all types of geographical data. It ties together spatial locations with descriptive attribute data.

Components of GIS

Components of GIS

  • Hardware: Computers, servers, scanners, plotters, GPS receivers.
  • Software: Processing applications (e.g., ArcGIS, QGIS).
  • Data: The core of GIS.
    • Spatial Data: Location information (Where is it?).
    • Attribute Data: Descriptive information (What is it? Size, name, population).
  • People: Users, analysts, developers who interpret the data.
  • Methods: Procedures and workflows for spatial analysis.

Data Models

Vector vs. Raster

  • Vector Data: Represents geographic features as discrete geometric objects defined by coordinates.
    • Points: Trees, wells, poles.
    • Lines: Roads, rivers, pipelines.
    • Polygons: Parcels, lakes, political boundaries.
    • Best for: Discrete features, property lines, network analysis (routing).
  • Raster Data: Represents geography as a continuous grid of cells (pixels), where each cell holds a value.
    • Examples: Satellite imagery, Digital Elevation Models (DEM), scanned historical maps.
    • Best for: Continuous phenomena (elevation, temperature gradients, land cover classification).

Spatial Analysis Operations

GIS Capabilities

  • Buffering: Creating a zone of a specified distance around a point, line, or polygon (e.g., finding all houses within 100 meters of a flood zone).
  • Overlay Analysis: Superimposing multiple data layers to identify relationships (e.g., overlaying a soil map with a property map to find parcels suitable for farming).
  • Network Analysis: Calculating the optimal route or flow through a network of lines (e.g., the fastest driving route or tracing a leak in a water pipe network).
Key Takeaways
  • Scale: f/(H−h)f / (H-h). Varies with ground elevation. Higher ground = larger scale.
  • Vertical Photo: Tilt <3∘< 3^\circ. Necessary for mapping and removing distortion.
  • Relief Displacement: Radial shift outwards for high objects. Can be used to measure height (h=dH/rh = dH/r).
  • LiDAR: Active laser system; crucial for mapping bare-earth topography under tree canopies.
  • GIS Data Models: Vector uses discrete coordinates (Points, Lines, Polygons). Raster uses continuous grids of pixels (Imagery, DEMs).
  • GIS Analysis: Goes beyond mapping to answer spatial questions (buffering, overlay).
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