Map Projections, Reference Maps, Positioning and Mapping Measurement Explained

1.5. In a Secant Cylindrical Transverse Projection, geographic coordinates are projected onto a plane wrapped around the Earth in the form of a cylinder that touches the Earth along a meridian (orthogonal to the equator). In a tangential projection, the diameter of the cylinder coincides with the diameter of the planet, whereas a secant cylinder is smaller than the diameter of the Earth, thus cutting through its surface along two meridians. Since projection scale is 1:1 at the tangent meridian, smaller than 1:1 in-between secant tangents, and larger than 1:1 outside tangents, the average projection error is smaller in the case of a secant projection, as compared with the tangential counterpart. When adapted to a specific region, the transversal cylinder is rotated to fit the region's central meridian. Hence, the tangent meridian of a tangential projection coincides with the central meridian, whereas the central meridian is located in-between secant meridians. With the curvature of Sweden being predominantly situated in the south-north direction, it is predominantly covered with a transversal cylindrical projection, and therefore holds a relatively small average projection error.

6. Formal Reference Maps A terrain map is typically constituted by landscape hydrology, land cover, infrastructure, urban structures, topography, etc. Where water is abundant, hydrology typically constitutes the map skeleton. In arid areas, this may be replaced with infrastructure. The referential skeleton is used for orientation – hydrologic landscape elements are the main geographic reference in a postglacial nation like Sweden. The terrain map is typically used for physical planning, geographic analysis, geographic presentation, and for geographic orientation.

7. In 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.

8. Mention At Least Two

• Passive infrared satellite-mounted sensors like Landsat, that 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.

9. 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.

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 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.

10. Measuring Length on a Plane

Choice of distance metric: with the Pythagorean metric being applicable when the shortest distance between two locations is being calculated on a flat plane, and with the great circle metric doing 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 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.