City of Sheridan : Introduction to Maps

T a b l e o f C o n t e n t s

All About Map Scale
Representative Fractions and Statements of Representation
Common Map Scales

Map Projections and Reference Systems
Ellipsoids, Spheroids, and Datums
Types of Map Projections
Common Map Projections and Reference Systems

Types of Maps
Types of Geographic Data | Types of Geographic Features | Types of Maps

What is GIS?
What is GPS?

What is a Map?

Definitions:

What is a map? A graphic depiction on a flat surface of the physical features of the whole or a part of the earth or other body, or of the heavens, using shapes to represent objects and symbols to describe their nature. Maps generally use a specified projection and indicate the direction of orientation. GIS Dictionary; http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway Any graphical representation of geographic or spatial information. GIS Dictionary; http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway A map is defined as a representation of the whole or a part of an area. Maps are usually prepared on a flat surface, but globes are also maps in that they represent the entire earth. Although we think of maps as depicting geographical areas, they can also be used to represent other areas, such as the heavens or parts of the human body. We therefore say that explorers map the earth, astronomers map the heavens, and geneticists map chromosomes The University of Texas at Arlington, Associate Director of Libraries Gerald D. Saxon; http://libraries.uta.edu/ccon/whatis.shtm Usually a two-dimensional representation of some spatial reality, showing selected natural or man-made features or data, preferably constructed on a definite projection with a specified scale. Geographic Information Systems, An Introduction, Jeffery Star and John Estes, Prentice Hall, 1990.

Characteristics of Maps

All maps have the following characteristics:

All maps are concerned with two primary elements:
1. The locations of features.
  Inherent to the locational aspects of features on maps are these technical issues:
  - Accuracy (See "Map Accuracy").
  - Resolution (See "Map Resolution").
  - Scale (See "All About Map Scale").
2. The information, data, or attributes associated with these features.
The transformation of a geoidal surface onto a two-dimensional plane
(See "Map Projections and Reference Systems").
The portrayal of information or data through symbology.
The abstraction of reality.

Map Accuracy

The accuracy of the information shown on a map is important. Consideration in determining what the map will be used for is given when a map is created. Using the map for something it was not intended to be used for could lead to incorrect conclusions on the results.

There are many factors which influence how accurately spatial features can be located on a map. Some of these are:

The quality of the source data.
The width of the lines and the size of the symbols used to depict features.
The scale of the map.

If you were drafting a map by hand at a scale of 1:24,000 the finest line you could draw would represent a corridor about 20 feet wide!. (This assumes a pen width of 1/100 of an inch wide. Since 1:24,000 is one inch on the map to 2000 feet on the ground, 1/100 of 2000 would be 20 feet. And if the pen width is greater than 1/100, the corridor depicted by the line would be even larger. See " All About Map Scale" for more information on this subject).

Depicting data on a map at a greater scale than the resolution at which it was developed also affects the accuracy of what you see. Keep in mind that while maps have constraints in terms of accuracy, they can still be useful for analysis and decision-support when properly used.

Map Resolution

Resolution is the degree to which closely related entities can be discriminated. Since a paper map is always the same size, its data resolution is tied to its scale. Resolution also limits the minimum size of feature that can be stored.

Generally, as the scale increases the resolution of features more closely resembles what is found on the ground, because the ground-to-map extent of reduction is less. Likewise, as scale decreases so does resolution. In this case, linear features such as roads or rivers must be displayed as a single line rather than with two lines, and small areas must be depicted as point symbols rather than as polygons.

Obviously, resolution and accuracy go hand-in-hand. Data cannot more accurately represent itself as the scale is increased. For example, displaying Bureau of the Census TIGER data at 1:24,000 does not yield a more accurate map than displaying it at 1:100,000, the scale at which such data is originally digitized.

What About Map Scale

Representative Fractions and Statements of Representation
The scale of a map may be referenced both as a representative fraction, and as a statement of equivalence (representation).
A representative fraction is simply the ratio of a unit on the map to a like number of units on the ground. This ratio is expressed as a fraction. For example, a representative fraction of 1:24,000 in inches means one inch on the map represents 24,000 inches on the ground. In the United States, representative fractions are usually expressed in inches.

Since 24,000 inches in the example above is the same as 2000 feet, we can also state that one inch on the map represents 2000 feet on the ground. This is commonly referred to as a statement of equivalence, although this is a misnomer. One inch can only equal 1/12 foot. Anything else is simply a representation. Thus, a statement of equivalence is better termed a statement of representation!

Larger versus Smaller Scale
1:2,400 is "larger" scale than 1:24,000, and 1:24,000 is "larger" scale than 1:100,000. Why? The amount of detail which can be "seen" at a particular scale is the key.
The larger the map scale, the greater the amount of detail which may be incorporated onto the display or page. More detail can be seen at 1:2,400 than at 1:24,000, and at 1:24,000 than at 1:100,000. Thus the larger the map scale, the smaller the value of the denominator in the representative fraction. In terms of area, as the scale gets larger, the area which may be viewed on a given display or page gets smaller.

Another way of looking at this is to say that in a Large Scale map things are lager (the scale number is small 1:1200) and on a Small Scale map things appear smaller (the scale number is lager 1:12000000). For example a building on a Large Scale map might show the outline of the building and on a Small Scale map this same building might only be shown as a dot.

Common Map Scales

Here is a comparison of some common representative fractions and their statements of representation:
Repres Fract Statement(s) of Representation in inches in feet in miles

1 :      100      1" =       8.33'
1 :      120      1" =      10'
1 :      240      1" =      20'
1 :      600      1" =      50'
1 :    1,000      1" =      83.33'
1 :    1,200      1" =     100'
1 :    2,400      1" =     200'
1 :    4,800      1" =     400'
1 :    6,000      1" =     500'
1 :    6,336      1" =     528'        1" =  0.10 mi
1 :   10,000      1" =     833.33' 
1 :   12,000      1" =   1,000'
1 :   15,840      1" =   1,320'        1" =  0.25 mi
1 :   24,000      1" =   2,000' 
1 :   31,680      1" =   2,640'        1" =  0.50 mi
1 :   50,000      1" =   4,166.67'
1 :   63,360      1" =   5,280'        1" =  1.00 mi
1 :  100,000      1" =   8,333.33'     1" =  1.58 mi
1 :  126,720      1" =  10,560'        1" =  2.00 mi
1 :  250,000      1" =  20,833.33'     1" =  3.95 mi
1 :  316,800      1" =  26,400'        1" =  5.00 mi
1 :  500,000      1" =  41,666.67'     1" =  7.89 mi
1 :  633,600      1" =  52,800'        1" = 10.00 mi
1 :1,000,000      1" =  83,333.33'     1" = 15.78 mi
1 :1,267,200      1" = 105,600'        1" = 20.00 mi
1 :1,584,000      1" = 132,000'        1" = 25.00 mi
 

Map Projections and Reference Systems

Map Projections
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. It can be visualized as a transparent globe with a light bulb at its center (though not all projections emanate from the globe's center) casting lines of latitude and longitude onto a sheet of paper. Generally, the paper is either flat and placed tangent to the globe (a planar or azimuthal projection) or formed into a cone or cylinder and placed over the globe (cylindrical and conical projections). Every map projection distorts distance, area, shape, direction, or some combination thereof.
This is sort of like taking a basketball and cutting it up so it will lay flat on a table. There are many different ways to this and all will leave some distortion as they will not all lay perfectly flat and still align up with each other

There are many ways of projecting three-dimensional space onto a two-dimensional surface. Regardless of how the projection is done, a projection is really just a model for depicting the Earth's surface in a particular manner.

Reference or Coordinate Systems
Reference or coordinate systems allow for locating features on a surface based on specific measurements. There are two types of coordinate systems:

Planar Coordinate System.
Locations in planar geometry are often referenced through a Cartesian coordinate system. Cartesian coordinate systems are rectangular gridded networks of x,y values spaced equally across a surface. Locations are described across this network in terms of their x,y coordinate values. This system allows for consistent measures of angle, area, and length across the two-dimensions. Various mathematical algorithms are used to project three-dimensional surfaces into a coordinate system.

Perhaps the best known planar system is the State Plane Coordinate System (SPCS) in use in the United States. This system divides the U.S., Puerto Rico, and the Virgin Islands into over 120 numbered regions referred to as zones. The shape of the zone determines which projection the SPCS zone is built upon. The Transverse Mercator projection is used for zones which trend in an north/south manner, such as Wyoming. Zones in which the major axis trends east/west use the Lambert Conic Conformal projection, such as in Colorado. The panhandle of Alaska uses the Oblique Mercator projection.

The SPCS was developed in the 1930's by the U.S. Coast and Geodetic Survey using the North American Datum of 1927 (NAD27). It was designed to map the country at 1 : 10,000 accuracy, which was the limit of survey accuracy for the time.

Improvements in survey and Global Positioning System (GPS) technology required the development of a new datum for North America. The North American Datum of 1983 (NAD83) is now the standard for the SPCS.
NAD83 was also adopted by the state of Wyoming: Wyoming Statutes, Title 34, Chapter 25 - Plane Coordinates System

City of Sheridan GIS datasets currently use the State Plane Grid Coordinate System for the Wyoming East Central Zone (NAD83) U.S. Survey Feet. Transverse Mercator Projection.

Spheroid (Three-Dimensional Based) Coordinate System.
Coordinates in a spherical reference system use the measures of latitude and longitude. Any circle can be divided into 360 units called degrees. Traditionally, latitude and longitude are measured in degrees, minutes, and seconds.

Latitudes are horizontal lines which encompass the Earth parallel to the Equator and are measured in degrees north and south of the Equator. The Equator is 0 degrees and is no doubt the best known line of latitude. The Tropics of Cancer and Capricorn are also latitudes. The geometric quality of lines of latitude causes them to be referred to as parallels.

Longitude lines form great circles which encompass the Earth and pass through both the North and South Poles. Since all lines of longitude pass through the poles they tend to converge on each other as latitude increases. Lines of longitude are also commonly referred to as meridians. They are measured in degrees east and west of the Prime Meridian, which passes through Greenwich, England. Locally, 105 degrees west longitude passes through the Auraria Campus near downtown Denver, Colorado.

This spheroid-based system is commonly referred to as the Geographic reference system.

Since the geographic reference system is 3-D based, it requires a mathematical conversion in order to be depicted on a 2-D plane. Hence, the geographic coordinate system is not a map projection in and of itself. Similarly, neither is a Cartesian coordinate system such as the State Plane.
Ellipsoids, Spheroids, and Datums
Any map projection must be based on some geometrical and/or mathematical model or representation of the Earth's surface. But what model?
The Earth is often thought of as a sphere. In reality, there is a slight flattening of the Earth at the poles, and a slight bulging of the Earth at the Equator. Technically, the Earth is an ellipsoid.

The measure of flattening is called ellipticity. It is simply the difference in magnitude between the two axes of an ellipse. In the case of a sphere, ellipticity is 0.0. With the Earth, ellipticity is approximately 0.003353. Ellipsoids which approximate a sphere, such as the Earth, are also called spheriods.
Map Projections and Coordinate Management, 2nd Edition, Environmental Systems Research Institute (ESRI), March 1992.

Datums are simply a reference for modeling the Earth's surface based on some definition of the spheroid.

Properties of Map Projections
All map projections have the following properties:

The depiction of the Earth's surface onto a flat surface involves distortion of three or more of the following parameters:

Shape
Area
Distance
Direction
Note that no projection will correctly reflect all of the above criteria. A particular projection may preserve one property at the expense of the others, or it may compromise several properties with a reduction in accuracy.

Different projections produce different distortions.

The characteristics of any particular projection make it appropriate for certain uses and inappropriate for others.

Types of Map Projections
There are four common types of map projections:

Conformal
Conformal projections preserve the shape of localized regions. Area, however, may be greatly distorted.

Equal Area
Equal area projections preserve the area of displayed regions. However, shape, scale, and angle may have to be distorted to do this.

Equidistant
Equidistant projections preserve the distances between certain points. No projections, even equidistant, maintain distances (or scale) to or from all points on a map.

True Direction
A great circle is the shortest distance between two points on an ellipsoidal surface such as the Earth's. True direction or azimuthal projections rectify some great circle arcs, giving bearings or azimuths of all points on the map correct with respect to the center of the map.

There are three basic classifications of projection surfaces:

Conic
The simplest conic projection is tangent to the spheroid along a line of latitude known as the standard parallel for that projection. Meridians are then projected onto the conical surface. Parallel lines of latitude appear as rings as they are projected onto the cone. The cone is then cut along any meridian to produce the final conic projection. The effect of this is that meridians, which converge on Earth, appear as straight lines, while lines of latitude, which are parallel on Earth, appear as concentric arcs.
Other conic projections include secant and oblique.

Cylindrical
As with conic projections, cylindrical projections may have one line of tangency or two lines of secancy around the spheroid. The most common cylindrical projection is the Mercator which usually uses the Equator as its line of tangency.
Meridians are geometrically projected onto a cylinder, while lines of latitude are mathematically projected onto the same. The cylinder is cut along any meridian to produce a final projection. The effect is that meridians are equally distanced, while the spacing between lines of latitude increases as you approach the poles.

Planar
Planar projections project the spheroid onto a flat surface which is usually tangent to the spheroid (touches the spheroid at one point), but may be secant. These projections are also referred to as azimuthal or zenithal.
The point of tangency specifies the aspect of the projection. Functionally, the aspect serves as the focus of the projection. Common aspects for planar projections are polar, equatorial, and oblique.

Common Map Projections and Reference Systems
The following is a listing of common map projections and reference systems:
Coordinate or Reference Systems

Planar Coordinate Systems

STATE PLANE COORDINATE SYSTEM

Spheroid (Three-Dimensional Based) Coordinate Systems

GEOGRAPHIC (latitude, longitude)

Map Projections

ALBERS CONIC EQUAL AREA
AZIMUTHAL EQUIDISTANT
BIPOLAR OBLIQUE CONIC CONFORMAL
CYLINDRICAL EQUAL AREA
EQUIDISTANT CONIC
GNOMONIC {planar}
LAMBERT CONFORMAL CONIC
LAMBERT AZIMUTHAL EQUAL AREA
MERCATOR {cylindrical}
OBLIQUE MERCATOR {cylindrical}
POLYCONIC {conic}
ROBINSON {pseudo cylindrical}
TRANSVERSE MERCATOR {cylindrical}
UNIVERSAL TRANSVERSE MERCATOR (UTM) {cylindrical}

Note that there are many map projections which are not listed here.

Types of Maps

Types of Geographic Data
There are two types of geographic data which are typically represented on maps:

Continuous
Continuous data generally represents some type of surface. As such, the data has no distinct boundaries. Examples of data which are continuous in nature are slope, aspect, elevation, soils, soil contamination, noise, and radiation levels at a nuclear power plant. 5)

Discrete
Discrete data is characterized by having distinct nominal, ordinal, interval, or ratio values forming distinct boundaries. Discrete data is also known as noncontinuous or thematic data. Examples are political jurisdictions, land ownership, and zoning.

Types of Geographic Features
There are four types of geographic features that are often represented on maps:

Points
Point information is positional or place data representing spatial features which either have no meaningful area at the scale to be mapped, or for which information concerning the area or shape is lacking, with the result that the feature is best positioned by use of a point symbol.

Lines
Linear features are those such as roads or rivers. Although having width (and area) in the real world, these features are often mapped with a single line depending upon the scale at which depicted.

Areas
Areal information is often thought of as polygon-based data in that it has shape and area. Such data is generally mapped at a scale at which the boundary of the extent in question is discernable.

Surfaces
Surfaces are commonly represented as a set of contours or isolines representing lines of equal value, or as 2.5-D or 3-D models of data containing x,y and z values where z represents elevation or some other continuous data.

Types of Maps
There are many types of maps, and any given map is often a hybrid of one or more of the following or other types. Here we will mention three:

General Reference (Base)
General reference maps are also commonly known as base maps. Typically these maps depict the spatial relationships of various physical or man-made features or phenomena. Features which are often displayed include:

Boundaries
Hydrography (water)
Hypsography (elevation contours)
Public Land Survey System (PLSS)
Roads
Topographic maps are a special type of base maps which deals primarily with depicting physical and important man-made features on the surface of the Earth.

Thematic
The spatial variation or distribution of some type of information forms the emphasis for thematic maps.

Navigational Charts
Navigational charts refer to specialized maps generated for nautical or aeronautical purposes, although many reference maps are used for navigation as well. For example, 1:24,000 scale U.S.G.S. topographic maps are commonly used by mountaineers for orientation and route-finding.

What is GIS?

Definitions:

GIS
Acronym for geographic information system. An integrated collection of computer software and data used to view and manage information about geographic places, analyze spatial relationships, and model spatial processes. A GIS provides a framework for gathering and organizing spatial data and related information so that it can be displayed and analyzed.
GIS Dictionary; http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway
A GIS is a computer system capable of capturing, storing, analyzing, and displaying geographically referenced information; that is, data identified according to location. Practitioners also define a GIS as including the procedures, operating personnel, and spatial data that go into the system.
USGS GIS Poster; http://erg.usgs.gov/isb/pubs/gis_poster

Many people who have Internet access have used GIS. Some examples include going to a web site and typing in locations to determine how you will get from one place to another, or just to see where something is on a map. Some of these types of sites include:
YAHOO Maps
MSN Maps
Google Maps
MapQuest Maps

Another site that has a more interactive type system is Google Earth, a download which installs software on your system and runs data through the internet.
Google Earth

There are then free software's you can download and then download data to go with them to do some basic GIS type work. They are limited in their abilities but will do some basic GIS work and make some basic maps. An example of this is:
ArcExplorer

More advanced GIS software is made to do the basic mapping and analysis as well as in depth analysis. This might include analysis such as proximity of events, slope or elevation differences, capacities or volumes, routing or traffic control, water and sewer monitoring, and many other uses. This type of processing requires Methods, People, Hardware, Data, and Software to carry out.

What is GPS?

Definitions:

GPS
Acronym for Global Positioning System. A system of geosynchronous, radio-emitting and receiving satellites used for determining positions on the earth. The orbiting satellites transmit signals that allow a GPS receiver anywhere on earth to calculate its own location through triangulation. Developed and operated by the U.S. Department of Defense, the system is used in navigation, mapping, surveying, and other applications in which precise positioning is necessary.
GIS Dictionary; http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway

GPS is a system of satellites and receivers that allow devices to pinpoint their location on the earth. The heart of the system relies on 24 satellites that orbit the earth twice per day. The first GPS satellite was launched in 1974 and the 24th was launched in 1994. The system is operated by the United States Department of Defense and use of the system is free for anyone. New satellites are periodically launched to replace aging ones.

GPS is also a tool used by surveyors for accurately locating points on the earth's surface by using satellites in space. This is done through specific and tested methods to determine specific accuracy. The equipment used for this is designed for survey grade accuracies.

GPS is a tool used to collect data for the use in GIS and in making maps. The equipment used here is usually not as accurate as survey grade equipment. For the most part GIS uses equipment in the area of SUB-METER (+/- 3 feet).

Both Surveying and GIS (sub-meter) units can use correction signals to improve the accuracy to the level they use. Some use real time correction signals and others use what is called Post Processing.

GPS is also used by recreational users. GPS unites are being marketed to every niche possible, cars, boats, hunting, fishing, hiking, pda's, and phones to name a few. A recreational unit generally does not use a correction signal and is limited in accuracy. Prior to the turning off of selective availability (SA) on May 2, 2000, the accuracy of any single GPS unit (survey or not) was approximately 100 meters. Once selective availability was turned off the accuracy of any single GPS unit was reduced. This reduction varies and is estimated to be 10-20 meters. With Wide Area Augmentation System (WAAS) accuracies have improved for single GPS units. Reports indicates WAAS accuracies are in the 3-5 meter range. Results very depending on satellites used proximity to WAAS signals and source locations.

With the turning off of SA new uses for GPS have emerged. Also new activities such as GEOCACHING have emerged. At the time this was compiled there were at least 5 Geocache points in the city of Sheridan and many more in the area.