Map Projection Terms.docx

**[IMPORTANT TERMS OF MAP PROJECTION]** 1. Map Projection 2. Graticule: It is the network of parallels and meridians represented on a certain scale. This is the mesh by which the maps of the world or a part of it is represented. 3. Generating Globe: It refers to the globe from which pr...

**[IMPORTANT TERMS OF MAP PROJECTION]** 1. Map Projection 2. Graticule: It is the network of parallels and meridians represented on a certain scale. This is the mesh by which the maps of the world or a part of it is represented. 3. Generating Globe: It refers to the globe from which projections are generated or developed. Normally, it is a skeleton globe of glass or wire according to a given scale. 4. Developable Surface: It is the surface by which the 3 dimensional globe can be represented on a 2 dimensional surface. It is of two types: a. Cone b. Cylinder a. Conical Projection- Conical Projection with one and two standard parallels, Bonne's Projection, Polyconic Projection etc. b. Cylindrical Projection- Cylindrical Equal Area Projection, Cylindrical Equi-Distant Projection, Mercator's Projection etc. 5. Standard Parallel: This is the parallel on which the projection plane i.e. developable surface is being tangent upon. The scale of the projection remains perfect on this parallel. Therefore, the error is minimum along this parallel. 6. Central Meridian: This is the central most meridian on which the radial scale remains perfect. Beyond this meridian, the scale is being distorted towards east or west. 7. Constant of Cone: It is defined as the ration between the angle at the vertex or apex of a cone when developed (α) and the angle of the pole of the generating globe. Mathematically, it can be expressed as under: Constant of Cone (n) = α/360 It varies between o and 1. 8. Scale Factor: It is the ratio between the denominator of the principal scale and the denominator of the real scale Scale Factor= Principal Scale/Real Scale It is of two types: a. Tangential Scale Factor (TSF): It is the ratio of the length of the parallel on the globe (Lpg) and the length of the parallel on the map (Lpm)as represented as under: TSF= Lpg/Lpm b. Radial Scale Factor (RSF): It is the ratio of the length of the meridian on the globe (Lmg) and the length of the meridian on the map (Lmm)as represented as under: RSF= Lmg/Lmm 9. Datum: It is the representative level on which the heights of the points of the earth's surface are based upon. Generally, it is the mean sea level of any country. Earlier, the datum was highly variable and it varied from one country to another. But at present, to avoid multiple datum around the globe, one single datum is used which is known as WGS-84 i.e. World Geodetic Survey, 1984. 10. Geoid: It is the shape of the earth which not exactly a sphere. Little bit of undulations are found and therefore, the geoid does not actually match the sphere. **ORTHODROME & LOXODROME (Great Circle & Rhumb Line)** A **great circle**, also known as an **orthodrome**, of a [sphere](https://en.wikipedia.org/wiki/Sphere) is the intersection of the sphere and a [plane](https://en.wikipedia.org/wiki/Plane_(geometry)) that passes through the center point of the sphere. A great circle is the largest circle that can be drawn on any given sphere.  [great circle](https://en.wikipedia.org/wiki/Great_circle), which is the path of shortest distance between two points on the surface of a sphere. On a great circle, the bearing to the destination point does not remain constant. If one were to drive a car along a great circle one would hold the steering wheel fixed. Therefore, great circle is a line of constant change of bearing. **Loxodrome** is a curve which cuts every member of a system of lines of curvature of a given surface at the same angle. A ship sailing towards the same point of the compass describes such a line which cuts all the meridians at the same angle. In Mercator\'s Projection (q.v.) the Loxodromic lines are evidently straight. If one were to drive a car along a rhumb line, one would have to turn the wheel, turning it more sharply as the poles are approached. Therefore, rhumb line is a line of constant bearing.