This annex is intended for use as a guide to spatial address encoding within SDTS.
Spatial address refers to the geographic point location of an object, and is used to define the position of a point (in a defined coordinate system) that may be on, above, or below the Earth's surface. There are many systems available for indicating point locations. This standard allows for the use of any of the three most widely used in the United States (listed in order of preference): Latitude and Longitude, Universal Transverse Mercator/Universal Polar Stereographic (UTM/UPS) Grid Systems, and State Plane Coordinate Systems (metric). These systems are mathematically interconvertible, on a point by point basis, and are also officially recognized by many mapping and surveying agencies of the Federal and state governments.
Use of altitude data is not required; however, specifications for altitude data are provided in 4.1.3.5.5, Altitude.
References for more detailed information on the methodology, techniques and applications of the three systems are provided in 1.3.
When transforming the coordinates of linear objects (for the purposes of using this standard or otherwise), great care must be taken. A line segment (the most simple component of most of the linear objects) is defined as "a direct line between two points." For the purposes of this standard, a direct line shall be defined as a line of constant slope (the change in Y or latitude divided by the change in X or longitude). When the points of a line segment are specified by coordinates within a given coordinate system, the direct line between them is a function of only that coordinate system. This means that when a line segment is transformed, there will always be two direct lines defined by that line segment: one line before transformation and the second line after transformation. If the distance between the two points of a line segment is great enough, the two direct lines could be significantly different. If this is the case, a data encoder using this standard should break the line segment into two or more segments (by adding intermediate points) to ensure that the resultant before and after sets of direct lines are not significantly different.
The problem of transforming arc objects is conceptually even greater than that of line segments. Just as with line segments, points provide the basis for positioning the arc, and these points can be transformed just as line segment points. But whereas "direct line" has a well defined meaning in both a before and after transformation reference surface, a "curve that is defined by a mathematical function" might not. It can be said that there are no general solutions available for transforming curves.
However, from a practical standpoint, the problem might not be so great. When dealing with large scale data (within a relatively small area), where arc objects are most likely to be used (e.g., highway construction and land parcel maps), and rectangular coordinate projection systems are used, the before and after transformation differences are usually not significant. Where this is not the case (differences are significant), this standard requires that a data encoder convert the arc to a string (or chain if appropriate) with enough line segments to ensure proper relative positional accuracy is retained. The fact that this conversion has been done should also be available to a user of the encoded data.
Latitude and longitude are ellipsoidal coordinate representations that show locations on the surface of the earth using the earth's equator and the prime meridian (Greenwich, England) as the respective latitudinal and longitudinal origins.
Latitude and longitude are angular quantities, and according to the standard, should be expressed as decimal fractions of degrees.
Degrees of latitude, according to the standard, should be represented by a two-digit decimal number ranging from 0 through 90.
Degrees of longitude, according to the standard, should be represented by a three-digit decimal number ranging from 0 through 180.
When a decimal fraction of a degree is specified it, according to the standard, should be separated from the whole number of degrees by a decimal point.
Latitude north of the equator, according to the standard, should be specified by a plus sign (+) or by the absence of a minus sign (-), preceding the two digits designating degrees. A point on the equator, according to the standard, should be assigned to the northern hemisphere. Latitude south of the equator shall be designated by a minus sign (-) preceding the two digits designating degrees.
Longitudes east of the prime meridian, according to the standard, should be specified by a plus sign (+) or by the absence of a minus sign (-), preceding the three digits designating degrees of longitude. Longitudes west of the meridian, according to the standard, should be designated by a minus sign preceding the three digits designating degrees. A point on the prime meridian, according to the standard, should be assigned to the eastern hemisphere. A point on the 180th meridian shall be assigned to the western hemisphere.
Any spatial address with a latitude of +90 or -90 degrees specifies the location of the North or South pole, respectively. The longitude component may have any legal value.
The Universal Transverse Mercator Grid System (UTM) provides rectangular coordinates that may be used to indicate locations of points on the surface of the Earth. UTM involves linear measurements, and the unit of measure is the meter. A point is located by specifying a hemispheric indicator, a zone number, an easting value, and a northing value.
UTM is designed for world use between 80 degrees south latitude and 84 degrees north latitude. The globe is divided into narrow zones, 6 degrees of longitude in width, starting at the 180 degree meridian of longitude and progressing eastward. The zones are numbered 1 through 60. Each zone has, as its east and west limits, a meridian of longitude. Each zone also has a central meridian passing through the center of the zone. The location of any point within a zone is given in relation to the central meridian within that zone and the equator. The system zone yields positive values for the identification of a point on the earth's surface by first assigning numeric values to the equator and the central meridian. Then, a point's north-south location is obtained by either adding or subtracting the point's distance north or south of the equator. Similarly, a point's east-west location is obtained by either adding or subtracting the point's distance east or west of the central meridian.
A value of 500,000 meters is assigned to the central meridian of each zone in order to avoid negative numbers at the west edge of the zone. The values increase from west to east. For north-south values in the northern hemisphere, the equator is assigned 0 meters, and the numbers increase toward the north pole. In the southern hemisphere, the equator is assigned 10,000,000 meters, and the numbers decrease toward the south pole.
On a map, appropriate values for the easting and northing of a point are determined relative to labeled grid lines. A point on the equator is assigned a value of zero for its northing and is treated as if it were in the northern hemisphere. A point on a boundary meridian is assigned the zone number for the zone to the east of the point.
The Universal Polar Stereographic Grid System (UPS) is used in place of UTM in the polar regions of greater than 84 degrees north latitude and 80 degrees south latitude. Characteristics of UTM (paragraph C.2.1) also apply to UPS, with some important modifications. The 0 degree and 180 degree meridians divides each polar region into an eastern and western half. At the north pole, the western grid zone is labeled "Y" and the eastern grid zone "Z". The corresponding south polar grid zones are labeled "A" and "B". The location of any point within either of the polar regions is given in relation to the 180 degree meridian and 90 degree meridian. A value of 2,000,000 meters north and 2,000,000 meters east is added in order to avoid negative numbers.
The first graphics character of the Zone Number subfield in the External Spatial Reference module record shall be a code to indicate the hemisphere in which the point is located. A plus sign (+), according to the standard, should be used to indicate the northern hemisphere, and a minus sign (-) to indicate the southern hemisphere. The remainder of the subfield, according to the standard, should contain the zone number indicating the 6 degree longitudinal band in which the point is located (01, 02, ... 60) for UTM or grid zone letter designator (A,B,Y or Z) for UPS.
The unit of measurement for both Northing and Easting, according to the standard, should be the meter.
The State Plane Coordinate Systems (SPCS's) are designed to define the location of points within a geographic grid system. They were used first in the nineteenth century; the first formal use was in 1932. There are now one or more State Plane Coordinate Systems in use in each of the 50 United States, as well as in the Commonwealth of Puerto Rico, the U.S. Virgin Islands, American Samoa, and Guam. The District of Columbia is included with the State of Maryland. State Plane Coordinate Systems represent separate, distinct systems for the 54 political jurisdictions involved, as opposed to the universally applicable Latitude and Longitude and Universal Transverse Mercator/Universal Polar Stereographic (UTM/UPS) Grid Systems.
Nine States, Puerto Rico, American Samoa, and Guam are covered individually by one State Plane Coordinate System or zone. The nine States are: Connecticut; Delaware; Maryland; New Hampshire; New Jersey; North Carolina; Rhode Island; Tennessee; and Vermont. The remaining 41 States and the Virgin Islands are covered individually by from two to ten SPCS's. These systems fall into four general categories, based upon the conformal mapping projection methods utilized in the political jurisdiction:
A zone may extend to the State boundaries of a political jurisdiction and to county boundaries where these are contiguous with State boundaries.
Further, a zone may be defined in one of three ways. In each of these three methods, an arbitrary point of origin in latitude and longitude is one element of the definition of the zone. The other element of definition varies with the conformal mapping projection system used in the zone:
The arbitrary point of origin for each zone is typically located outside the geographic area it covers. This is designed to meet the objective that no coordinate may have a negative value.
Each of the zones or SPCS's in each jurisdiction is uniquely identified by a four-character numeric code as specified in Table 4 of FIPSPUB 70-1. This four character code, according to the standard, should be transferred in the Zone Number subfield of the External Spatial Reference module.
Three methods are available for the designation of the east-west (X or E(easting) coordinate) and north-south (Y or N(northing) coordinate) location indicators: (1) the Lambert Conformal Conic Projection, (2) the Transverse Mercator Projection, and (3) the Oblique Mercator Projection used in Alaska.
An X or Y coordinate in an existing SPCS may be expressed by a number of the general magnitude of NNNNNNN.NNN. This will suffice for a range of not less than .003 meters and not more than 3,000,000.000 meters, and is considered to be appropriate for this standard. For the purposes of coordinate transfer the following conventions apply. Where a decimal fraction is used, according to the standard, it should be one, two or three positions in length, as required (for example, .1, .15., .125).
The unit of measurement of both the X and Y coordinate, according to the standard, should be the meter, as required by the SPCS '83 specification.
Currently, many of the SPCS's are, through legislative mandate, referenced to the North American Datum Adjustment of 1983 and are referred to as SPCS 83 systems. The SPCS systems must have their horizontal components expressed in meters to conform to the SPCS specifications. However, there remain several SPCS's which are referenced to the North American Datum of 1927 and are referred to as SPCS 27 systems. The SPCS 27 specifications required that the horizontal components be expressed in feet. The horizontal component of SPCS 27 data must be converted to meters.
Altitude of a point, as used in this standard, is defined as the distance in meters either above or below a reference surface. The Vertical Datum and Sounding Datum subfields of the External Spatial Reference module, according to the standard, shall be used to specify this surface.
All altitude measurements below the reference Vertical Datum, according to the standard, should be designated by a minus sign (-) preceding the number. Measurements at or above the Vertical Datum, and at or below the Sounding Datum, may be either without a sign or may be designated by a plus sign (+); however usage, according to the standard, should be consistent throughout a set of data.
The use of meters as the unit of vertical measurement is required.
The spatial address field can accommodate additional dimensions. For each added dimension, the Spatial Address field will have one subfield added. The label for the new SADR subfield will match the Dimension Label subfield in the corresponding Internal Spatial Reference record. The format for the new SADR subfield shall match the Format subfield defined in the corresponding Dimension Definition record. The units for the new SADR subfield will be described by the Dimension Value Measurement Unit subfield of the corresponding Dimension Definition record. The number of subfields in the Spatial Address field will be equal to the number of geospatial dimensions defined in the Internal Spatial Reference module plus the number of nongeospatial dimensions defined by the Dimension ID fields of the Internal Spatial Reference module record. The order subfields will appear in the Spatial Address is determined by the order of repetition of the Dimension ID fields.
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