Maps have also some errors give reason
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HomeContact Infomation
Making a map requires several steps that include:
Selecting a geodetic datum (e.g. an idealized geometric description of the Earth’s shape used to define longitudeand latitude).Picking a projection method used to transpose geographic features, defined by points of longitude and latitude, from a curved earth’s surface (defined by the geodetic datum) to a flat paper surface.Choosing an appropriate scale to print the map.Data collection.
Each of these steps can be expected to introduce errors that will influence how hikers use maps to navigate the backcountry.
Geodetic Datum Considerations
While maps can be based on any one of a hundred or so existing geometric descriptions of the earth’s average shape called a geodetic datum (e.g. the shape, size and orientation in space of areference ellipsoid) in practice only 2 datums are important to Washington State hikers. The first, and still very commonly used datum(e.g. USGS Topographic Maps), is the North American Datum of 1927 (NAD 27) which was created using measurements taken over North America relative to a reference point very near the geographic center of the forty-eight Contiguous United States (Meades Ranch;Kansas) to create a simple geometric model of the earth. Thus, NAD27 is a local referencing system which should only be used when making maps of North American. The newer datum is the World Geodetic System of 1984 (for our purposes GRS 1980 and NAD83 datums are just WGS84 with a different name). The WGS84 datum was generated from worldwide satellite data, uses the center of the earth has an origin, and is a reference ellipsoid that best fits an idealized sea level (a.k.a. geoid) worldwide. Thus WGS84 can be used to define longitude and latitudeanywhere in the world and WGS84 is the default standard datum for GPS units. While perfectly good hiking maps for Washington can be made using either NAD27 or WGS84 errors come into play when map or GPS information from NAD27 and WGS84 are incorrectly combined. In Washington the difference between WGS 84 and NAD 27 can be over 200 meters. It is also important to remember that all datums use an idealized mathematical abstraction of the physical Earth that is much smoother than the earth’s physical surface . For example, before plotting locations in Mt. Rainier National Park on a map they are first projected onto the datum (e.g. a smooth ellipsoid) and elevation relief is lost. Thus if one calculates the distance from Paradise to the summit of Mt. Rainier by only measuring the lateral distance on a map a significant underestimation of the true distance will be made. To reduce this error you would have to include vertical information in the calculation of distance (e.g. extract elevations from contour lines and calculate the distance along a profile)
Projection Considerations
The Earth is curved and maps are flat. Map projection is the mathematical process of flattening out the curved Earth onto a flat piece of paper or computer monitor. While in theory there is an infinite number of possible map projections there are only a few that hikers of Washington State would typically encounter. These projections include:
.
HomeContact Infomation
Making a map requires several steps that include:
Selecting a geodetic datum (e.g. an idealized geometric description of the Earth’s shape used to define longitudeand latitude).Picking a projection method used to transpose geographic features, defined by points of longitude and latitude, from a curved earth’s surface (defined by the geodetic datum) to a flat paper surface.Choosing an appropriate scale to print the map.Data collection.
Each of these steps can be expected to introduce errors that will influence how hikers use maps to navigate the backcountry.
Geodetic Datum Considerations
While maps can be based on any one of a hundred or so existing geometric descriptions of the earth’s average shape called a geodetic datum (e.g. the shape, size and orientation in space of areference ellipsoid) in practice only 2 datums are important to Washington State hikers. The first, and still very commonly used datum(e.g. USGS Topographic Maps), is the North American Datum of 1927 (NAD 27) which was created using measurements taken over North America relative to a reference point very near the geographic center of the forty-eight Contiguous United States (Meades Ranch;Kansas) to create a simple geometric model of the earth. Thus, NAD27 is a local referencing system which should only be used when making maps of North American. The newer datum is the World Geodetic System of 1984 (for our purposes GRS 1980 and NAD83 datums are just WGS84 with a different name). The WGS84 datum was generated from worldwide satellite data, uses the center of the earth has an origin, and is a reference ellipsoid that best fits an idealized sea level (a.k.a. geoid) worldwide. Thus WGS84 can be used to define longitude and latitudeanywhere in the world and WGS84 is the default standard datum for GPS units. While perfectly good hiking maps for Washington can be made using either NAD27 or WGS84 errors come into play when map or GPS information from NAD27 and WGS84 are incorrectly combined. In Washington the difference between WGS 84 and NAD 27 can be over 200 meters. It is also important to remember that all datums use an idealized mathematical abstraction of the physical Earth that is much smoother than the earth’s physical surface . For example, before plotting locations in Mt. Rainier National Park on a map they are first projected onto the datum (e.g. a smooth ellipsoid) and elevation relief is lost. Thus if one calculates the distance from Paradise to the summit of Mt. Rainier by only measuring the lateral distance on a map a significant underestimation of the true distance will be made. To reduce this error you would have to include vertical information in the calculation of distance (e.g. extract elevations from contour lines and calculate the distance along a profile)
Projection Considerations
The Earth is curved and maps are flat. Map projection is the mathematical process of flattening out the curved Earth onto a flat piece of paper or computer monitor. While in theory there is an infinite number of possible map projections there are only a few that hikers of Washington State would typically encounter. These projections include:
.
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