Throughout the globe this scale is constant. So, if that's the case, then if this scale on our map is the same as the scale on a reference globe and we divide one scale by the other, if they're the same, then our scale factor is going to be one and the scale factor is just a way of quantifying or describing distortion and I'm just trying to put it in simple terms is that if you have the scale on your mapping the same as the scale on the reference globe, there's no distortion and the way we describe that as saying the scale factor equals one. [18], Latitude = (90 noon altitude + declination), The declination of major stars are their angles north and south from the celestial equator.

As a result of preserving angles and shapes, area or size of features are distorted in these maps. There were some variations. In transverse aspect of conical projections, the axis of the cone is along a line through the equatorial plane (perpendicular to Earths polar axis). The cylinder may be either tangent or secant to the reference surface of the Earth. As an example, the actual scale at a given point on map with scale factor of 0.99860 at the point and nominal map scale of 1:50000 is equal to (1:50000 x 0.99860) = (0.99860 / 50000) = 1:50070 (which is a smaller scale than the nominal map scale). So, here we have the cylinder touching our reference globe at the equator, and so the line where those two touch each other is the standard line and on our two-dimensional projected version of this map that's our standard line there. Historically, the average error in the determination of longitude was much larger than the change in position. And the oblique aspect refers to the cylinder being centered along a great circle between the equator and the meridians with its orientation at an angle greater than 0 and less than 90 degrees relative to the Earths axis.

The cylinder is then cut lengthwise and unwrapped to form a flat map. So, it's better to use a bar scale on larger scale maps where the scale is more consistent across the mapped area. And for the oblique aspect, the plane surface has an orientation between polar and transverse aspects. Keep in mind that while some projections use a geometric process, in reality most projections use mathematical equations to transform the coordinates from a globe to a flat surface. Examples include Lambert Azimuthal Equal-Area, Gnomonic, and azimuthal equidistant projections. It is also modeled more accurately as an oblate spheroid or an ellipsoid. http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html [16], Henry D. Thoreau classified this true meridian versus the magnetic meridian in order to have a more qualitative, intuitive, and abstract function. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. http://www.youtube.com/watch?v=AI36MWAH54s&feature=related [17], The meridian passage is the moment when a celestial object passes the meridian of longitude of the observer. This course is for anyone who wants to learn about mapping and GIS. Gall-Peters cylindrical equal-area projection Tissot's indicatrix On our map though, there can be distortion and so the local scale is the scale on that map which may or may not be the same as on the reference globe around the principle scale. The course includes both practical software training and explanations of the concepts you need to know to make informed decisions as you start your journey to becoming a GIS analyst. As mentioned above, a reference globe (reference surface of the Earth) is a scaled down model of the Earth. (software is not provided), Geographic Information System (GIS), Cartography, Esri, Mapping, Spatial Analysis. The areas of features on the map are proportional to their areas on the reference surface of Earth. http://www.youtube.com/watch?NR=1&v=EPbQQNrBIgo When the sun passes two times an altitude while rising and setting can be averaged to give the time of meridian passage. The resulting map plane in most instances can be rolled around the globe in the form of cylinder, cone or placed to the side of the globe in the case of the plane. Maps do not suffer from the above shortcomings and are more practical than globes in most applications. The cone may be either tangent to the reference surface along a small circle (any circle on the globe with a diameter less than the spheres diameter) or it may cut through the globe and be secant (intersect) at two small circles. In planar (also known as azimuthal or zenithal) projections, the reference spherical surface is projected onto a plane. The cone can be situated over the North or South Pole. During such transformation, the angular geographic coordinates (latitude, longitude) referencing positions on the surface of the Earth are converted to Cartesian coordinates (x, y) representing position of points on a flat map. UTM - Definition, Facts & Location - Video & Lesson Transcript | Study.com", "Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes", "Research on magnetic declination in Lithuanian territory", "What is the difference of noon position and meridian passage? Globes are not suitable for use at large scales, such as finding directions in a city or following a hiking route, where a more detailed image is essential. Along the standard parallel lines in this map (45 N and 45S), there is no scale distortion and therefore the ellipses would be circular. So, if you're dividing, remember you're just dividing one by the other and so when you do that, you're going to end up with a value of two at that part of the map. So, the scale on the globe being the same as the scale on the map, where will that happen? The reason the standard line is important, is that's where there was no distortion, you're literally transferring a point from the reference globe to the sheet of paper and if there are touching each other then that is the same point on both of those things and so there can't be any distortion because you actually haven't shifted it or moved it in any way. They are used in thematic mapping. That's a key idea. Map Scale True-direction projections are used in applications where maintaining directional relationships are important, such as aeronautical and sea navigation charts. So, we're only going to have the principle scale along our standard line. Similarly, when trying to project a spherical surface of the Earth onto a map plane, the curved surface will get deformed, causing distortions in shape (angle), area, direction or distance of features. One way of classifying map projections is by the type of the developable surface onto which the reference sphere is projected. http://www.quadibloc.com/maps/mapint.htm [15], The true meridian is the chord that goes from one pole to the other, passing through the observer, and is contrasted with the magnetic meridian, which goes through the magnetic poles and the observer. Mercator - conformal projection Tissot's indicatrix [13] Navigators were able to use the azimuth (the horizontal angle or direction of a compass bearing)[14] of the rising and setting Sun to measure the magnetic variation (difference between magnetic and true north). So, for example, if at this location, we have a scale of one to 50 million instead of the principal scale which is one to 100 million, which is what we would have at the standard line, then we have one over 50 million divided by one over 100 million and that equals a value of two.

Directions are true from the center point (North Pole). http://www.csiss.org/map-projections/, USGS Decision Support System: http://mcmcweb.er.usgs.gov/DSS/ A report was submitted to the "Transactions of the Royal Society of Canada," dated 10 May 1894; on the "Unification of the Astronomical, Civil and Nautical Days"; which stated that: The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the horizontal component of magnetic force lines along the surface of the earth. Well, it's going to be at the standard line, it's going to be where the sheet of paper is touching the globe. On a curved surface, measuring terrain properties is difficult, and it is not possible to see large portions of the Earth at once. Overview of the Meridian Distance Approximations", "The Prime Meridian: Establishing Global Time and Space", "What is the Prime Meridian? They are expensive to produce, especially in varying sizes (scales). Great circles are the shortest distance between two points on the surface of the sphere (known as great circle route). The developable surface serves as a good illustrative analogy of the process of flattening out a spherical object onto a plane. A common method of classification of map projections is according to distortion characteristics - identifying properties that are preserved or distorted by a projection. Some projections do not preserve any of the properties of the reference surface of the Earth; however they try to balance out distortions in area, shape, distant, and direction (thus the name compromise), so that no property is grossly distorted throughout the map and the overall view is improved. I am looking forward to the remaining courses in the field. an orange peel) onto a flat surface without some stretching, tearing, or shearing. Circular shapes of the same size indicate preservation of properties with no distortion occurring. A clever way of illustrating the amount of distortion is taking place on a map is using a variable scale bar which is particularly popular with the Mercator projection because it works well for that. Latitude and Longitude A classic example of area exaggeration is the comparison of land masses on the map, where for example Greenland appears bigger than South America and comparable in size to Africa, while in reality it is about one-eight the size of S. America and one-fourteenth the size of Africa. [22][23][24] Although, a few time zones are offset by an additional 30 or 45 minutes, such as in the Chatham Islands, South Australia and Nepal. It is often used for mapping Polar Regions (with the source located at the opposite pole). where is there no distortion. Equidistant projections are used in air and sea navigation charts, as well as radio and seismic mapping. The cone is cut lengthwise and unwrapped to form a flat map. The choice of a projection for a map depends on such factors as the purpose for which the map will be used, the area being mapped, and the maps scale (distortion is more pronounced in small-scale mapping). Meridians and parallels are perpendicular to each other. The plane is tangent to North or South Pole at a single point or is secant along a parallel of latitude (standard parallel). In the secant case, the cylinder intersects the globe; that is the diameter of the cylinder is smaller than the globes. Transverse and oblique aspects are seldom used. So, if you're making a map of Canada or United States or some area that size, there's no point really in putting a scale bar on there because of the amount of distortion is taking place. So, this is the calculation that gets done. The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian. These are lines of equidistance. This indicates that any line joining north and south poles (meridian) is true to scale and therefore distances are accurate along these lines. They are also used in atlases and thematic mapping. Topo Map Slope The two axes of the ellipse indicate the directions along which the scale is maximal and minimal at that point on the map. The adoption of World Geodetic System 84" (WGS84) as the positioning system has moved the geodetic prime meridian 102.478 metres east of its last astronomic position (measured at Greenwich). [7][unreliable source?] I think it may be more clear to you now that the amount of distortion that takes place can have an effect on the way that scale is perceived by somebody when you're looking at your map. So, what's happening is they're saying that if you're at this latitude that is 600 statute miles, if you're at this latitude that is 600 statute miles it's still 600 miles. A globe is a scaled down model of the Earth. The resulting map from this projection looks like a globe (similar to seeing Earth from deep space). The meridian passage can occur within a few minutes of variation. For the polar or normal aspect, the cone is tangent along a parallel of latitude or is secant at two parallels. central meridian). Contour Lines - Terrain 2