GIS Chapter 2

• The central meridian of a UTM zone has a scale factor of

0.9996

• List the four types of map projections by the preserved property

1) Conformal Projection- preserves local angles and shapes. 2) Equivalent Projection- represents areas in correct relative size. 3)Equidistant Projection -maintains consistency of scale along certain lines. 4) Azimuthal Projection- retains certain accurate directions. (subsection under Azimuthal Projection) - a) Lambert Conformal - conic projection. B) Albers equal-area - conic projection

• The secant case means that a cylindrical projection has _____ line(s) of tangency

2

• Which of the following statements is true? A coordinate system is based on a map projection. Each UTM zone covers ______ degrees in longitude

6

Explain how a UTM zone is defined in terms of its central meridian, standard meridian, and scale factor

A UTM zone is mapped onto a secant case (secant case - line that intersects a curve at a minimum of two distinct points) transverse Mercator projection, with a scale factor of 0.9996 at the central meridian. The standard meridians are 180 kilometers to the east and west of the central meridian. Each zone is 6 degrees. There are 60 zones.

What is a datum?

A datum represents the Earth as a mathematical model, which serves as the reference or base for calculating the geographic coordinates. Geographic coordinate systems use a spheroid to calculate positions on the earth. A datum defines the position of the spheroid relative to the center of the earth.

A geographic coordinate system definition

A geographic coordinate system is a reference system that uses latitude and longitude to define the locations of points on the surface of a sphere or spheroid. A geographic coordinate system (GCS) includes a DATUM, PRIME MERIDIAN, and ANGULAR UNIT.

What is a map projection?

A map projection is defined as a systematic transformation of the coordinates (latitude and longitude) on the surface of a sphere (or ellipsoid) into plane coordinates on a flat surface (x, y). A method by which the curved surface of the earth is portrayed on a flat surface. This generally requires a systematic mathematical transformation of the earth's graticule of lines of longitude and latitude onto a plane. A map projection is a systematic arrangement of parallels and meridians on a plane surface. A map projection transforms the earth surface, based on an elliptical, to a plane Locations of features on a map are based on a plane coordinate system or a Cartesian CS.. Expressed in x- and ycoordinates. Locations of spatial features on the Earth's surface are based on a GCS expressed in longitude and latitude. A map projection bridges or links the two types of CS. Projection converts data sets from GCS to projected CS. Every map projection distorts 1. distance, 2. area, 3. shape, 4. direction, 5. or some combination

How does an ellipsoid differ from a sphere in approximating the shape and size of the Earth?

A three-dimensional, closed geometric shape, all planar sections of which are ellipses or circles. An ellipsoid has three independent axes, and is usually specified by the lengths a,b,c of the three semi-axes. If an ellipsoid is made by rotating an ellipse about one of its axes, then two axes of the ellipsoid are the same, and it is called an ellipsoid of revolution, or spheroid. If the lengths of all three of its axes are the same, it is a sphere.

Describe the three types of map projections by the projection or developable surface.

Cartographers will use a globe and a geometric object to show how to make a map projection. The three different types of map projections are conic, cylindrical, and azimuthal. When a map projection is labeled a cylindrical projection, it can be made using a cylinder. A conic projection map projection is made using a cone and azimuthal projection uses a plane.

• Which of the following spheroids is ground-measured, rather than satellite-determined?

Clarke1866

• How can "datum shift" affect GIS work?

Datum shift can result in substantial shifts of positions of points.

• Describe how on-the-fly projection works

Designed for displaying data sets that are based on different CS. The GIS software uses the projection files available and automatically converts the data sets to a common CS.By default the CS, is the same as CS of the first data set in the display. If the data set has an unknown coordinate system, the GIS software use an assumed CS such as NAD27 and the assumed geographic coordinate system.

• An X-shift of -500,000 means you add 500,000 to the original X coordinate value. (True or False?)

False

Explain the difference between NAD27 and NAD83

In North America, people who use GIS will use NAD27 and NAD83. NAD27 is a predefined coordinate system and stands for North American Datum of 1927. This datum is based on the Clarke 1866 ellipsoid, which originates in Meades Ranch in Kansas. NAD83 stands for the North American Datum of 1983 and is based on the GRS80 ellipsoid. NAD83 is different from NAD27 because it is a geocentric datum reference to the center of Earth's mass.

Explain the importance of map projection

Map projections change geographic coordinates on an ellipsoid to locations on a plane. Usually, projection is one of the initial tasks to be done in a GIS project. Map projections provides some advantages such as allowing the use of two-dimensional maps and allows people to work with plane coordinates instead of values of longitude and latitude.

• Which of the following statements is true?

Meridians are lines for measuring location in the E-W direction, and parallels are lines for measuring location in the N-S direction.

An example for South Central Texas

NAD 1983 UTM Zone 14N

What are two common datums used in the US?

NAD27 and NAD83

UTM grid system may be based on

NAD27, NAD83, or WGS84.

17. Describe how on-the-fly projection works.

On-the-fly projection is used for showing data sets that are based on different coordinate systems.

• Which of the following statements is true about North American Datum (NAD)?

Only NAD83 is a newer datum than NAD27 and NAD83 is based on a satellite-determined spheroid.

• The Geographic Coordinate Data Base (GCDB) is a database based on the

PLSS (Public Land Survey System)

Describe the four types of map projections by the preserved property

Preserved property is divided into the following four classes- conformal, equal area or equivalent, azimuthal or true direction, and equidistant.

15. Which UTM zone are you in? Where is the central meridian of the UTM zone?

San Antonio is located in UTM Zone 14N. The central meridian of this UTM zone is 99 degrees W.

*Standard Meridians are what distance from the Central Meridian?

Standard Meridians are 180 Kilometers to the east and the west of the Central Meridian

What is Web Mercator?

The Web Mercator is now the standard projection for online mapping like Bing Maps, Google Maps, and MapQuest. The Web Mercator is slightly different from the Mercator projection, however, Web Mercator uses spherical formulas with every scale. It uses the Mercator on a sphere and from the coordinates from WGS84, it projects from latitude and longitude. It is a conformal projection, which preserves shapes and local angles and also has the top of the map as north. The Web Mercator does have distance distortions at high latitudes just like the Mercator projection.

•What are Easting and Northing?

The X value, called the Easting, has a value of 500,000m at the central meridian of each zone (Figure 2). The Y value, called the Northing, has a value of 0 m at the equator for the northern hemisphere (Figure 3), 10,000,000 m at the equator for the southern hemisphere.

• Which of the following statements is NOT true about a meridian with a scale factor of 1?

The meridian must be the line of 00 longitude.

How is the scale factor related to the principal scale?

The scale factor is the ratio of the local scale to the principal scale where the principal scale is taken from the ratio of the radius of the globe to the radius of the Earth. The scale factor is the normalized local scale.

Explain the difference between the standard line and the central line

The standard line is the line of tangency between the reference globe and the projection surface. There is NO projection distortion along a standard line. The central lines (I.e., the central parallel and meridian) define the center of a map projection.

Name two commonly used projected coordinate systems that are based on the transverse Mercator projection

The two commonly used projected coordinate systems that are based on the transverse Mercator projection are Universal Transverse Mercator (UTM) grid system and State Plane Coordinate (SPC) System.

• When converted from NAD27 to NAD83, horizontal shifts of point positions in the conterminous United States can be as much as 100 meters (328 ft). (T/F)

True

• Name the ellipsoids (or spheroids) that are the basis for NAD27, NAD83, and GPS, respectively

WGS84

• Which datum are GPS readings based on?

WGS84

What is WGS84?

WGS84 is the World Geodetic System 1984. WGS84 is the datum for GPS readings. WGS84 is a satellite determined ellipsoid.

• Which coordinate does a false easting apply to?

X

When converted from DMS to DD units, 46°30'00'' will read

answer is 46.5°

The center of a map projection is determined by the

central parallel and central meridian

Projection

converts data sets from GCS to projected CS.

• What is it meant by "reprojection"?

converts from one projected CS to another system.

The UTM (Universal Transverse Mercator) coordinate system general information to know

divides the world into sixty north-south zones, each 6 degrees of longitude wide. UTM zones are numbered consecutively beginning with Zone 1. Each zone is further divided into northern and southern hemispheres. The designation of a UTM zone will carry a number and a letter. By using a projection for each UTM zone, accuracy is maintained by at least one part in 2500. Which means that a distance measured over 2500 meter course on the UTM grid system would be accurate within a meter of the true measure. In the Northern hemisphere, UTM coordinates are measured from a false origin located at 500,000 meters west of the UTM zone's central meridian. In the Southern hemisphere, UTM coordinates ae measured from a false origin located at 10,000,000 meters south of the equator and 500,000 meters west of the UTM zone's central meridian. The use of false origins, means that UTM coordinates are all positive.

Scale factor

is the normalized local scale, defined as the ratio of the local scale to the principal scale.

Conformal projection

keeps local shapes and angles.

• The longitude reading of a point in Oregon should be entered as a _______ value in a GIS package

negative: negative value

• Which of the following coordinate systems is treated as a predefined coordinate system in ArcGIS?

only UTM (Universal Transverse Mercator) and STP (State Plane)

A conformal projection preserves the property of

preserves local angles and shapes

Azimuthal projection

projection retains certain accurate directions.

Equivalent projection

represents areas in correct relative size.

ArcGIS offers the following three methods for defining a coordinate system

select, import, or create a coordinate system. Explain the difference between select and import a coordinate system.

Describe the three levels of approximation of the shape and size of the Earth for GIS applications

sphere, spheroid or ellipsoid, and geoid. If you view Earth from space, it looks like a perfect sphere, but it's actually not! It's wider along the equator than between the poles. An approximation of the earth shape is spheroid or ellipsoid. An ellipsoid has it's major axis along the equator and its minor axis connecting the poles.

• The two common map projections used for the SPC (State Plane Coordinate) system are

transverse Mercator and Lambert conformal conic

• A map projection transforms the geographic coordinates on an ellipsoid into locations on a plane. (T/F)

true

• All layers to be used together in a GIS operation must align spatially (T/F)

true

Difference between vector and raster data

vector is discrete and raster is continuous.

Equidistant projection

will maintain scale consistently along standard lines.