Principles of photogrammetry
Photogrammetry means getting measurements from photographs. Take two photos of the same ground from two places. Anything tall appears at slightly different spots in the two photos. This difference is parallax. The taller the object, the bigger the parallax. From parallax, camera height and distance between the camera positions we can compute heights and make 3D models.
Photos are taken with about 60 percent forward overlap so that every part of the ground appears in at least two photos.
Using photogrammetry
- Drawing maps and contour lines of land.
- 3D models of buildings, mines and heritage sites.
- Finding volumes, such as a heap of sand or soil.
- Checking progress of a road or building site with drones.
Principles and types of remote sensing
Remote sensing means collecting information about the ground without touching it. Energy (usually sunlight) reaches the ground, each material reflects or gives out energy in its own pattern, and a sensor records it. Healthy plants reflect a lot of near-infrared light; water absorbs it and looks dark.
Types: passive sensors use sunlight or heat from the earth (cameras, thermal); active sensors send their own signal and listen to the echo (radar, lidar). Platforms can be ground, drone, aircraft or satellite.
Using remote sensing
- Crop health and yield estimates.
- Flood, fire and drought watch.
- Land-use change, forest cover and city growth.
- Weather forecast and sea surface temperature.
Principles and role of GIS
A GIS (geographic information system) stores data about places in layers, each with one theme. Data are in two forms: raster (grid of cells, like a photo) and vector (points, lines and polygons). Every feature has a location and a table of facts (attributes). Because all layers share the same position system, they can be stacked, searched and combined.
Using GIS
- Choose a site: overlay land use, roads and flood zones.
- Find the nearest hospital or best bus route.
- Manage water pipes, power lines and land records.
- Plan disaster relief.
For more detail see the lessons on Remote sensing and on GIS.
Key formulas and definitions
- Parallax: taller object = larger shift between photos
- Forward overlap is about 60 percent
- Raster = grid of cells; vector = points, lines, polygons
- Passive sensor: uses sunlight; active sensor: sends its own signal
Worked examples
1. Why do two overlapping photos help find the height of a tower?
The tower top shifts more between the photos than its base. This shift (parallax) grows with height, so measuring it gives the height.
2. A drone photo is 80 m wide on the ground. For 60 percent forward overlap, how far must the drone move between photos?
Overlap 60 percent means 40 percent is new. New part = 0.40 x 80 = 32 m. The drone moves 32 m.
3. A thermal sensor and a radar: which is active?
Radar is active, it sends a signal and listens for the echo. A thermal sensor is passive.
4. A city wants a school at least 500 m from a main road, on safe land, near houses. Which GIS tool helps?
Overlay the layers for roads (with a 500 m buffer), flood zones and houses. The place that meets all rules is the answer.
Common mistakes
- Thinking photogrammetry needs only one photo. Height needs two views.
- Thinking remote sensing always means satellites. Drones and aircraft are also used.
- Mixing raster and vector: raster is cells, vector is points, lines and polygons.
- Believing a GIS is only a map picture. It is a database with places.