Geographic Information Systems (GIS) Exam Notes

A geographic datum combines a position (from a geodetic datum) and an attribute value (describing geographic data at that position).

A problem that uses geographic location to solve issues.

Simple structure, stores continuous data (like temperature), and represents fuzzy objects (like wetlands).

No topology (connectedness), large storage space.

Precise geometry, multiple attributes per object, well-defined topology.

Unreasonable precision, complex algorithms, can't represent continuous things.

Spatial changes over a period of time

Calculating differences between consecutive snapshots in a time series detects changes.

An image corrected for perspective distortion, appearing as if viewed directly from above. It removes the visual effect of looking at a surface from an angle.

Visualizations of geographic information without formal mapping conventions, used for analysis and understanding, not precise location.

Map created adhering to cartographic standards for accurate geographic representation, used for reference and navigation.

Map specifically designed for maritime navigation, focusing on water features, coastlines, and sea depths.

Map emphasizing general geographic features and land formations, used for basic identification and information.

Map highlighting a specific theme or attribute, like population density, rainfall, or vegetation patterns.

A method for estimating values at unknown locations based on the weighted average of surrounding known values, where closer points have more influence.

In inverse distance weighting, it means that the influence of surrounding data points is the same in all directions from the unknown point.

The geoid helps define a local geodetic datum by providing a gravity reference and approximating the Earth's surface. A local ellipsoid is used to match the geoid's curvature locally, allowing objects on the Earth's surface to be projected onto the ellipsoid, resulting in a mathematically well-defined surface that maintains both the gravity reference and connection to the global ellipsoid.

Secant map projections offer a lower average scaling error compared to tangential projections. This is because they intersect the Earth's surface at two points, reducing distortion over a wider area.

The Universal Transverse Mercator (UTM) system utilizes a transversal secant cylinder projection that rotates 60 times to cover the Earth's circumference. This provides a reduced average scaling error compared to other projections.

There are five main GPS technologies: absolute code in real-time (absolute GPS), relative code in real-time (differential GPS), relative phase-shift with post-calculation (statistical GPS), relative phase-shift in real-time (single station RTK), and compensated relative phase-shift in real-time (network RTK).

Real-time GPS provides continuous position updates while moving, making it ideal for navigation and applications requiring dynamic positioning.

Non real-time GPS (statistical GPS) allows for highly accurate position calculations after long periods of stationary measurement, achieving sub-centimeter precision.

Ortho-correction eliminates geometric distortions caused by sensor perspective in aerial imagery, producing a georeferenced image where distances and shapes are accurate.

Ortho-correction involves adjusting image pixels based on elevation data and sensor position to remove perspective distortions and align the image with a map projection.