Taken from "Nebraska Geographic Information Systems Steering Committee -
Gis Technical Standards" December 15, 1993, version.


Spatial data conversions standards

(Obj 3 Task 2 - Identify and implement spatial data conversion standards)

     Coordinate systems are the foundation upon which the spatial portion of geographic 
information is established.  Coordinate systems are the means of referencing geographic 
information to locations on the earth's surface and are the reference to standard models of the 
earth's surface represented as oblate spheroid of revolution (a flattened sphere).  Data 
conversion as it applies to GIS is the conversion from one system and/or projection to another. 

     The National Geodetic Survey and the United States Geologic Survey (USGS) have 
developed computer programs for transforming data from one coordinate system to another. 
These programs include the General Cartographic Transformation Package (GCTP) and the 
North American Data Conversion package (NADCON).  These programs are widely available 
and have been embedded in a variety of commercial software packages.

     Map projections  deal with two largely abstract notions: the precise shape of the earth 
and the location in space of the plane that is to receive the "projected" information.  This 
projection process is more complicated by the fact that the earth is not a perfect sphere.  The 
earth is a spheroid that is compressed roughly along its north/south axis with bulges at the 
equator.  The distortions that accompany the transformation of geodetic coordinates 
(coordinates on the surface of earth geoid) to a cartesian or planar coordinate system (the 
simple x,y coordinates of the map sheet) will always compromise one or more of the principal 
spatial aspects of shape, area, distance or direction.  Figure 1 shows how geodetic coordinates 
are projected onto the map plane.  It is easy to see how a distance on the earth's surface can be 
either shorter or longer, relative to a fixed reference point, depending on the type of projection 
used and how the map plane is oriented to the earth geoid.

     Imperfections in shape of the earth, or the ability to mathematically model that shape, 
can affect the resulting transformation of coordinates.  The ability to accurately model the 
earth's shape and determine its center are sources of considerable debate, attention, and 
consternation in the lives of those who would make maps or otherwise work with geographic 
information.  All of this important to GIS because these computerized systems don't work 
directly with data expressed in geodetic coordinates.  They require that data be converted into 
a projection.  The choice of which projection system to use will have important consequences 
for the validity of any analysis that is subsequently done.  Given that GIS systems organize 
data into layers, themes, or coverages, it is critical to ascertain the projections and scale of the 
sources for these data so that all coverages can be transformed into a common projection and 
scale.  Without this information, the resulting coverages will not line up or "register" with 
each other.  Thus a line on one coverage that represents a portion of a county boundary that is 
supposed to coincide with a certain river may not line up correctly.

Figure 1: Projection of geodetic coordinates onto a map plane.




Figure 2: Commonly used map projections.



     As was mentioned above, projections result in distortions in the properties of distance, 
shape, area and direction.  Different projections will be better for an accurate representation of 
one or more of these properties so that careful consideration should be given to the purpose to 
which the data will be put.  For example, if the resulting data will be used for aircraft 
navigation, a projection that minimally distorts distance and direction along the principal axis 
of travel should be used.  Some projections are better suited for regions that are oriented along 
a north/south axis, such as Papio-Missouri River NRD, while others are more appropriate for 
east/west trending regions, such as the State of Nebraska.

     Figure 2 illustrates how the map plane can be formed into a variety of shapes, like a 
cylinder wrapped around the globe or a cone placed on the globe.  Some projections use a 
cone that has a section sunk below the surface of the Earth (the lines along which the cone's 
surface is secant with the Earth's surface are known as the principal meridians).  The 
important thing to remember is that regardless of what projection system is used, the Earth is a 
spheroid and any attempt to "flatten" its surface into a map will result in the already mentioned 
distortions of shape, distance and/or direction.  This is clearly apparent on most world maps, 
which are typically done in the Mercator projection.  On these maps, lands in the more 
southern or northern regions, such as Greenland, will appear much larger than they actually 
are.

     Scale is another important aspect of geographic information.  Scale is typically 
represented as a ratio between a unit of distance on the map and its equivalent real-world 
distance.  This a 1:24,000 scale map is sized so that a single unit of distance on the map is the 
equivalent of 24,000 units on the ground.  The scale at which geographic information is 
depicted and captured has a profound effect on the resolution or accuracy that can be achieved.  
The resolution of a map is the accuracy with which the location and shape of map features can 
be depicted for a given map scale.  The table below illustrates how the on-the-ground distance 
represented by a narrow line of 0.5 mm (0.0197 inch) width on a map varies at differing map 
scales:


          On-the-Ground Distance
     Map Scale     (0.5mm line width)
     1:500,000     250.00 m  (820.25 ft)
     1:250,000     125.00 m  (410.13 ft)
     1:100,000      50.00 m  (164.05 ft)
     1:63,360       31.68 m  (103.94 ft)
     1:24,000       12.00 m   (39.37 ft)
     1:12,000        6.00 m   (19.69 ft)
     1:600           0,30 m    (0.98 ft)

     As the scale of a map decreases (the ratio increases), long narrow features like streams 
and roads must be represented as lines and small areas become points.  These minimum sizes 
and dimensions are called the "minimum mapping units."  These minimum mapping units are 
usually incorporated into a set of rules for map construction.  For example, a mapper may 
decide that area features that are less than 1/8 inch on a side on the map will be represented as 
points and linear features less then 1/8 inch in width will be represented as lines.

     Factors relating to map scale, quality of source materials and field methods used to 
document the location of such features, map resolution, map maker, drafting skill, and the size 
of the instrument used to draw or render the map will all influence how accurately points and 
lines are placed on the map.  Map accuracy must be carefully considered, especially when 
combining coverages for analysis, as these errors are cumulative.  The manipulation of digital 
map data by the GIS in re-projecting the data or converting it for transfer to another GIS also 
introduces errors.

     A map plane is essentially a two-dimensional representation of a three-dimensional 
object, the earth's surface.  Regardless of what projection system is used, there is a measurable 
amount of distortion introduced in both distance and direction.  As long as the latitude-
longitude coordinates and the associated projection system are identified and specified in the 
database, it is immaterial as to what projection system was utilized.

     When spatial data is converted from one projection system to another, there is a potential 
of adding more error to the resulting data. Therefore, map projection or data conversion 
should always attempt to be made from the original database or map projection, if it is not 
available in the latitude-longitude coordinates form.

     The map projection software used can also have an effect on the amount of conversion 
error.  The National Geodetic Survey and the US Geological Survey have developed various 
standardized computer programs for transforming data that helps to minimize these errors.

     The internal representation of data in the computer also influences the amount of 
conversion error.  Some GIS systems allow data to be stored in either "single" precision or 
"double" precision accuracy.  Single precision is accurate enough for many types of data 
applications or uses, but some databases need to be stored in double precision to retain their 
original accuracy.


Recommendation:  The GIS Steering Committee recommends the conversion of GIS data 
should be done from the original data source with mapping software adhering to the NGS 
and USGS standards.  If the data were originally stored in double precision form then 
the conversion should also be produced with double precision accuracy.




Spatial data transfer standards

(Obj 3 Task 3 - Identify and implement spatial data transfer standards)

     Data transfer is the exchange of data and is essential to foster sharing of geographic data. 
Data transfer standards are operating rules that accomplish the transfer of data between 
incompatible systems and provide a common format for exchanging data from various software 
systems.  Data transfer standards provide a road map for interpreting various geographic data 
models.  Since each GIS contains its own unique, conceptual, logical, and physical data 
model, standards can provide a means for exchanging data between two GIS systems.  
Resolving characteristics of various spatial data models used by the different systems is a 
fundamental requirement for data transfer standards.

     At an abstract level, a GIS package supports at least one of the two spatial data models- 
the raster model and the vector model.  In the raster model, the information is provided in the 
form of images (or pixels).  In the vector model, the information is provided in the form of 
topology comprising of points, lines, and polygons.  A data format should adhere to any one 
of the two data models.

     Currently, the commonly used formats for raster data model are:

          ADRG          ARC Digitized Raster Graphics
          APIPS          ApplePips
          ASCII          ASCII text
          CCRS          Canadian Centre for Remote Sensing format
          DEM               USGS Digital Elevation Model
          DTED          Digital Terrain Elevation Data
          EOSAT          EOSAT Thematic Mapper floppy disk format
          EPPL7          EPPL7 format
          ERDAS          ERDAS ANT / DIG format
          GIF               Graphics Interchange Format
          GRASS          GRASS format
          GRD               Geosoft Grid file format
          GXF               Geosoft Grid eXchange Format
          IDIMS          IDIMS IDIPS format
          IGDS          Intergraph Graphics Design System
          MACPAINT     Macintosh MacPaint
          MPIPS          MicroPips
          NITF               DIA National Imagery Transfer Format
          OPTR9          OPTRONIX scanner format
          PCI               PCI image format
          PCIPS          IBM Personal Computer Image Processing System
          PCX               PCX format
          RAS               Houston Instruments Scanned Raster Format
          RDEF2          LT PLUS, DWRIS, LIDES Raster Data Exchange Format
          RIC               Scanners raster format
          RIE1               A one-byte Run Length Encoded Raster Format
          RLC               SCAN-CAD Run Length Encoded
          RSTR          A generic bit-mapped raster format
          SCAN-CAD     SCAN-CAD .IMG Format
          TERRA-MAR     Terra-Mar .IMG and .BIG
          TGA               AT&T Truevision
          TIFF               Tag Image File Format
          TIROS          TIROS/AVHRR format
          TM FAST          Eosat TM Fast Format previously copied from tape
          TM TIPS          Thematic Mapper TIPS format copied from tape
          VOHS          Paul Vohs' format

     The commonly used formats for the vector data model are:

          CFF               Cartographic Feature File Format
          COVERAGE     Arc/Info "Coverage" format
          DIME          GBF/DIME format
          DLG-STD          USGS Digital Line Graph STANDARD format
          DLG3-OPT     USGS Digital Line Graph OPTIONAL format
          DLG-E          Enhanced DLG
          DMDF          Digital Map Data Record Format
          DXF               AutoCAD Drawing eXchange Format
          EXPORT          Arc/Info "Export" format
          GENERATE     Arc/Info "Generate" format
          GERBER          GERBER file format
          GRASS          GRASS digit file format
          GSMAP          USGS digitizer format
          IDC               Polygon Information Overlay system format
          IGES               Initial Graphics Exchange Standard
          MAP               LT PLUS Vector Map file Format
          MMI               MapInfo Map Interchange Format
          MOSS          Map Overlay and Statistical System format
          Postscript          Postscript File Format
          SCITEX          SCITEX Response Graphics File
          SDTS          Spatial Data Transfer Standard
          SIF               Standard Interchange Format for film production
          SIF               on a SCITEX System
          SVF               Single Variable File Format
          TIGER          Census Bureau TIGER Line file format
          TYDAC          SPANS TYDAC VEH/VEC vector file format
          WDBII          World Data Bank II format



     To further discipline data transfer formats, the Spatial Data Transfer Standard (SDTS) 
has been designed by federal government agencies over a period of 10 years.  It provides a 
high-level model for geographic information with specifications for transferring data, encoding 
the transfer, and defining spatial features and attributes.  The Standard was officially approved 
by the Secretary of Commerce in July 1992 as a Federal Information Processing Standard, 
Publication 173 (FIPS 173) and provides the means to establish specific and consistent feature 
definitions.

     Data transfer can be done in any of the above formats.  Normally a format common to 
GIS systems is identified and used in data transfer.  However, in some cases the same format 
is implemented in different ways on different systems. This complicates the transfer of data.  
The SDTS was created to resolve some of these issues. The SDTS represents the most 
significant and comprehensive development to date of a standard for the transfer of geographic 
information between computer systems.  However the standard has not been implemented on 
most GIS systems.  Yet federal agencies have until February 1994 to implement the standard.  
When it is fully implemented and understood it should be the preferred format for most data 
transfers.


Recommendation:  The GIS Steering Committee recommends adoption of the Spatial 
Data Transfer Standards.