The Basics Datums Explained in More Detail The content of these two is very similar, but as their titles suggest, they supply different levels of information. As with all the Modules in this package, ICSM is trying to explain often complex situations using very simple language. In the case of Datums this is particularly true.
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. 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.Jul 12, · Vertical Datums. A vertical datum is a surface of zero elevation to which heights of various points are referenced.
Traditionally, vertical datums have used classical survey methods to measure height differences (i.e. geodetic leveling) to best fit the surface of the earth.
MX GPS Navigator Operator’s Manual M A D E T O M E A S U R E Page 2: Product Information. Operator’s Manual DGPS Navigator Version MX Series GPS MX MXB MX MXB MXBR Product Information The model and serial number of . A clear understanding of the different coordinate reference systems and datums in use today and the appropriate transformations between these is therefore essential to ensure rigorous.
University of Connecticut [email protected] Peer-reviewed Articles What Does Height Really Mean? Part I: Introduction Thomas H. Meyer University of . What we do. The mission of the National Geodetic Survey (NGS) is to define, maintain and provide access to the National Spatial Reference System (NSRS) (PDF, KB).The NSRS provides a consistent coordinate system that defines latitude, longitude, height, scale, gravity, and orientation throughout the United States and its territories.
What is meant by geodetic datum? (1 mark) Geodetic datums (1) were created to define the size and shape of the Earth and the origin and orientation of the coordinate systems used to map the planet. Hundreds of different datums have been used to frame position descriptions since the first es.