In order to use these angular displacements to understand distance relationships on the surface of the earth, you need to know the radius of the earth at the points being referenced. This would be simple if the earth was spherical; but it turns out that if you use this assumption, the distances that you calculate will have substantial errors. Since about it was postulated by Isaac Newton that the earth is not spherical and a few years layer this was confirmed by actual measurements of a degree of latitude in different parts of the world.
Preprocessing is an important but unheralded subject. It shapes the datasets you place in the GIS; it prepares them for analysis. This chapter looks at preprocessing the map or spatial component of your features.
This chapter begins with concepts that define the geographical referencing standards of the Earth. Topics include latitude and longitude, projections, coordinate systems, and datums. These concepts help you understand map preprocesses like changing projections, converting layers from vector to raster, and reclassifying or resampling layers.
A large part of map preprocessing is to make your data usable by providing consistent projection parameters throughout all your data sets. The goal is to make your layers fit properly over each other. Across the spherical Earth, latitude lines stretch horizontally from east to west left image in Figure 3.
Longitude lines, also called meridians, stand vertically and stretch from the North Pole to the South Pole center image in Figure 3. Midway between the poles, the equator stretches around the Earth, and it defines the line of zero degrees latitude left image in Figure 3.
Relative to the equator, latitude is measured from 90 degrees at the North Pole to degrees at the South Pole. The Prime Meridian is the line of zero degrees longitude center image in Figure 3.
An Overview of Geodesy and Geographic Referencing Systems. The magic of geographic information systems is that they bring together and associate representations from diverse sources and infer relationships based on spatial references. The tidal datums on this page are referenced to an arbitrary station datum. In order to apply these datums for surveying or coastal management they must be reduced to Mean Lower Low Water (MLLW), which is the reference datum for predictions, bench mark publication and nautical charting. In the past, military geodesy was largely involved with the practical aspect of the determination of exact positions of points on the earth's surface for mapping or artillery control purposes while the determination of the precise size and shape of the earth was a purely scientific role.
Longitude runs from degrees west of the Prime Meridian to degrees east of the same meridian. Because the globe is degrees in circumference, and degrees is the same location. In other words, small areas do not need a projection because the statistical differences between locations on a flat plane and a 3-dimensional surface are not significant.
For small-scale maps those that encompass a large area, see Figure 2. Our assumption that the Earth is round or spherical does not accurately represent it.
The Earth is a geoid with a slight pear shape; it is a little larger in the southern hemisphere and includes other bulges. The difference, however, between the ellipsoid and the geoid is minor enough that it does not affect most mapping. Until recently, projections based on geoids were rare because of the complexity and cost of collecting the necessary data to create the projection, but satellite imagery has helped with measurement and geoid projections are now more common.
We need flat maps. This reshaping cannot be done without introducing some error. To illustrate this point, imagine taking a cardboard globe, cutting it in half at the equator, and then cutting both the northern and southern hemispheres into four equal parts apiece.
Resting on a table, the pieces are not flat; they arch in the center. Try flattening one of the pieces. If you succeed, part of the cardboard will be scrunched together and other parts will tear apart.
By flattening it, you modify its geography. Map projections enable the reshaping of the Earth by mathematically transforming spherical coordinates x, y, and z to 2-dimensional x and y space. Different map projections cause different map distortions.
One way to classify map projections is to describe them by the characteristic they do not distort. Usually only one property is preserved in a projection. This chapter confines its focus to just two properties—area and shape—because the projections that preserve these properties—equal-area and conformal—are the most common.
Equal area or equivalent projections preserve the area or the amount of space within features. On a small-scale political map of the world, the areas within each country are preserved.
In reality, the area of Mexico and Greenland is similar, and in the right-hand map in Figure 3. Equal area projections, however, distort all the other properties. Shape, distance, and direction are not preserved.Tutorial Ellipsoid,geoid,gravity,geodesy,andgeophysics XiongLi Such an understanding is a geodetic Þction, invalid and unacceptable in geophysics.
Second, in gravity correc- geodetic purposes, but is completely inappropriate for geo-physics. In geophysics, gravity is used to learn about the den-.
Understanding Geodetic Datums and Its Purposes. 1, words. 8 pages. Important Things in Texas. Understanding Heights and Vertical Datums 45 minutes to 1 hour.
This lesson provides a basic understanding of vertical datums and how to choose the appropriate datum for a given application, with a conceptual introduction to ellipsoidal, geopotential, and tidal datums. Not understanding the difference between the old Datums (AGD66 ⁄ AGD84 or GDA94) and the Geocentric Datum of Australia (GDA).
Software settings on GPS receivers being ambiguous. This assists in creating confusion between what a projection does with what a datum does. Glossary-Dictionary of terms related to professional surveying, geomatics.. These terms are defined in accordance with their meaning and usage within the surveying profession.
Many of these terms are defined using slang or historical definitions. Prospective inbound mobility students can browse through the list of undergraduate courses available at UTM for the UTM Student Exchange Program below.