Geographic Problems in GIS (Exam 25 Mars,2014)

Geographic Problems in GIS

• Geographic problems involve location, either in the information used to solve them or in the solutions themselves.
• There's a need to categorize geographic problems due to their diverse nature.
• Three methods for categorizing geographic problems are scale, intent, and time-scale.

Scale or Geographic Detail

• Micro level (e.g., crystalline structure coordinates)
• Local level (e.g., cities, forests, parks)
• Regional level (e.g., Baltic runoff, nationwide flu epidemic)
• Global level (e.g., global warming, bird flu pandemic)

Intent or Purpose

• Practical/urgent (e.g., money, emergency)
• Curiosity driven (e.g., continental drift, historic migration patterns)

Time-Scale

• Operational (e.g., smooth functioning of an organization)
• Tactical (e.g., forest harvesting plan)
• Strategic (e.g., organizational long-term direction)

Geographic Data (Datum)

• Most basic form of geographic data links a position (in space and time) to descriptive properties (attributes).
• Geographic attributes are classified according to their associated scale.

Representing Geographic Data

• Two primary methods used to simplify geographic reality for computer representation:
  • Spatial averaging by pixel (tessellation)
  • Using the fact that many attributes remain constant over large areas.
• Spatial averaging: attribute values within a square pixel are set to a constant value, often the average value.

Fuzzy Geographic Objects

• Many geographic objects are fuzzy, meaning their boundaries are not clearly defined and may change over time.
• Examples include deltas and wetlands.
• Representing fuzzy objects often requires different methods than representing clearly defined objects.
• A good example is the representation of wetlands using a gradient or index of wetness that can be defined with multiple categories.

Secant Cylindrical Transversal Projection

• A secant cylindrical projection is a map projection in which a cylinder intersects the Earth's surface along two meridians.
• This type of projection is well-suited to countries that are predominantly north-south oriented (like Sweden).
• The average projection error is smaller in secant projection than in tangential projection.
• "Central meridian" in projection refers to the reference meridian.

Terrain Map

• The terrain map represents landscape hydrology, land cover, buildings and infrastructure, topography, etc.
• The skeleton of a terrain map can sometimes be the hydrologic landscape elements, in particular in post glacial countries.

Remote Sensing Sensors

• Passive sensors utilize external energy sources (like sun's radiation) or blackbody radiation to detect the reflection of energy.
• Examples: infrared satellite-mounted sensors (like LANDSAT), conventional digital cameras in airplanes, producing highly resolved reference information.
• Active sensors emit their own energy signal to detect the reflected signal.
• Examples: RADAR sensors to depict elevation, SONAR to depict seafloor elevation.

Choropleth Maps

• Choropleth maps represent data through polygons in which each polygon holds a constant attribute value.
• Examples include population density, or election results.

Multivariate Representation

• A multivariate representation allows multiple attribute values to be carried by a single symbol.
• An example is a naval chart where arrows represent currents, showing speed by their color and direction by their orientation.

Measuring Distance on Ellipsoidal Earth

• Errors arise when measuring distances on a plane projection of an ellipsoid Earth due to several sources:
• Choice of distance metric: choosing between Pythagorean, and Great circle metric.
• Projection methods and map scale have a substantial effect on results.
• Error magnitude also depends of the degree of geographic generalization.