GIS Data Input: Digitization, Scanning & GPS Data Collection
Idea in plain words
Data Input Methods: Manual digitizing, heads-up (on-screen) digitizing, scanner rasterization, direct GPS field collection, and remote sensing ingestion.
Exam tip
High-yield exam topic (High).
Explanation
Data acquisition represents up to 70-80% of the cost and duration of GIS projects. Historical paper maps must be scanned and georeferenced using Ground Control Points (GCPs) with known geographic coordinates. Once georeferenced, technicians trace points, lines, and polygons using heads-up digitizing. In the field, handheld GNSS receivers capture location data. Differential GPS (DGPS) compares satellite signal travel times against a known fixed benchmark, broadcasting real-time correction factors to mobile rovers to achieve sub-meter cadastral survey accuracy.
Key points
- Heads-Up (On-Screen) Digitizing: Tracing vector features directly over georeferenced aerial/satellite base maps on a computer monitor.
- Scanning: Converting hardcopy paper maps into high-resolution digital raster images via flatbed or drum scanners.
- Global Positioning System (GPS / GNSS): Satellite-based constellation measuring pseudoranges to compute exact 3D coordinates (X, Y, Z).
- Differential GPS (DGPS): Corrects satellite ephemeris and atmospheric timing delays using a stationary base station, reducing positional error from meters to centimeters.
Example
Example
Survey Department of Nepal digitizing 1:25,000 scale topographic maps into layered vector datasets (transportation, hydrology, contours).
Code examples
/*
GIS Data Conversion Pipeline:
[Paper Maps] -> [High-Res Scanner] -> [Raw Raster Image]
|
(Georeferencing via GCPs)
v
[Projected Georeferenced Raster]
|
(Heads-up Vector Digitizing & Snapping)
v
[Vector Layers: Points, Lines, Polygons]
*/
Exam questions
- [8 Marks] Explain the process of Heads-Up (on-screen) digitizing. How is a scanned paper map georeferenced using Ground Control Points (GCPs)?
- [6 Marks] Describe the working principle of Differential GPS (DGPS). How does it achieve centimeter-level accuracy for cadastral surveying?