GPS Positioning, Remote Sensing and Cartographic Measurement Explained

Geographic Positioning

Single-station RTK uses the GPS satellites in combination with a GPS transmitter installed at a known reference position, whereas network RTK operates with GPS satellites in combination with multiple reference stations. With a collation of reference stations covering a region, they provide spatially varying compensation regarding atmospheric sources of positional error, such as ionic activity and electric conductivity. SWEPOS is the national Swedish network of positional reference stations.

At Least Two Different Types of Sensors

Passive sensors utilize the reflectance of external energy sources (like the Sun) or blackbody radiation, whereas active sensors utilize the reflectance of a signal emitted by the sensor system itself.

• Passive infrared satellite-mounted sensors like Landsat, which are used for depicting vegetation.
• Conventional passive digital cameras mounted in airplanes for producing highly resolved reference information (on changing landscapes).
• Active airplane-mounted radar sensors for depicting elevation.
• Active ship-mounted sonar sensors for depicting sea-floor elevation.

What Is a Choropleth Map?

A choropleth map holds a polygon representation where each polygon holds a constant attribute value. For example, in the respective cases of nominal and ratio scales, a county map of Sweden, or a county-wise representation of inhabitant numbers, are typical choropleths.

Multivariate Representation

In multivariate representation, individual symbols carry multiple attribute values. An example might be a naval chart where current is represented with a field of arrows that are coded for speed with their respective colours, and for direction with their respective orientations.

Cartograms

Cartograms are maps that lack planimetric correctness and distort area or distance in the interest of some specific objective. An everyday example is underground or bus-line cartograms, where dwindling lines have been straightened out for the sake of clarity.

Measuring Length

Choice of Distance Metric

The Pythagorean metric is applicable when the shortest distance between two locations is being calculated on a flat plane, whereas the great circle metric does the same job on the surface of an ellipsoid. The latter produces an arc when projected onto a plane. Hence, the shortest distance across a plane map projection follows an arc, where the distance difference depends on the projection method and map scale.

Whatever metric is chosen, measurements are most often made on a horizontal projection of a three-dimensional world. Length and area may therefore be substantially lower than on a three-dimensional surface, where error magnitude depends on landscape steepness. The length of a true curve is always longer than the length of its polyline or polygon representation, where error magnitude depends on the degree of geographic generalization.