Data Standards - Recommendations
 

Insuring data interoperability is a critical task that should not be overlooked. Incompatible data is a thief of time, of human and financial resources, and of usefulness of your system. Developing and maintaining standards internal to an organization is a critical start to any data management project. Furthermore, when data is generated outside of an organization, measures must be taken to ensure that it can be integrated into the existing infrastructure. The goal of this section is to offer the user descriptions and recommendations for standardizing certain key elements involved with GIS and Spatial Data.

Industry standards have been developed by large scale data producers, data users, and government commissions over the years to maintain quality and insure data interoperability. These standards are not unified, as organizations and individual regions have specific needs that are only addressed locally. The following documents attempt to explain both the technical and political spatial data parameters that require attention to standards.

Standards are important for the Demining Community, because they provide an infrastructure for the exchange and usage of spatial data. Standards related to Geographic Information Systems and spatial data are very important issues when it comes to the transfer and compatibility of different data sets. In order for two sets of spatial data to be compatible the projection, reference ellipsoid and datum must be identical for each data set, or be able to be translated. It is also necessary to know which file formats can be used with which GIS packages and how to transfer that data between disparate computer systems.

 

File Formats

A file format refers to the specific way in which the raster or vector data is stored. It is necessary to know what file format the data is in so the user can determine if that data can be used with a particular GIS package.

The following is a listing of some common file formats used in GIS:

ADRG - ARC digitized raster graphic
ADRI - ARC interchange raster image file
GeoTIFF - TIFF based interchange format for georeferenced imagery
JPG - Joint photographic experts group format
IMG - ERDAS Imagine native format
ARC - ARC export file
DXF - Drawing exchange format
DLG - Digital line graph file
E00 - ArcInfo export format
GRID - ArcInfo GRID files
SHP - ESRI shapefile

 

Map Projections

Explanation: 

A map projection is a way to represent the 3-dimensional surface of the Earth on a 2-dimensional surface. There are many different map projections in use today and each one distorts some aspect of distance, direction, scale, or area. Some projections fit some areas of the Earth better than other areas and each one has a different purpose. Refer to the Map Projection section for a more detailed definition.

Recommendation: 

The Universal Transverse Mercator Projection (UTM) is the recommended projection to be used. This projection defines positions worldwide using the Transverse Mercator Projection. Transverse Mercator maps are often used to portray areas with larger north-south than east-west extent. Distortion of scale, distance, direction and area increase away from the central meridian. But with the Universal Transverse Mercator projection the earth is divided up into zones. Each zone is six degrees wide and has a central meridian in the center of the zone. Having different zones each with its own central meridian helps reduce possible distortions.

Alternative: 

It is not always possible to use the UTM projection and in that case a local projection that best fits the area of interest should be used. Often times the available data will be in this projection. This information can be found in the spatial reference information section of the data's metadata or in the bottom right hand corner of a map sheet.

Example:

Each of the following maps represents a different projection. If these were three different data sets from the same geographic location they would not overlay properly. For more examples of map projections and overlaid datasets that do not have the same projection please refer to the Map Projection standards section.

 

Geodetic Datums

Explanation: 

A geodetic datum is a model of the Earth's shape. It defines the location and orientation of a reference ellipsoid to the Earth. There are three main types of datums: local, regional and global. For example, the European Datum of 1950 is a regional datum and can be used for many areas in Europe including Norway, Portugal, Spain, and Sweden. Refer to the Geodetic Datum standards section for a more detailed explanation.

Recommendation: 

The World Geodetic System of 1984 (WGS 84) datum and its associated ellipsoid (WGS 84) is recommended for use because it is a common global system. It has worldwide coverage and this is the datum that the Global Positioning System is based on. Local datums can be converted to the WGS 84 datum.

There are a number of software packages that can be used to perform conversions between datums and coordinate systems. One of them is DatumPro available for purchase from Linden Software Limited, Geographic Translator by Blue Marble Geographics.

Alternative: 

It is not always possible to use the WGS 84 datum and in that case a datum that best models the Earth's shape for a particular area of interest should be used. The available data may already be in this datum. This information should be found either in the spatial reference information section of the data's metadata or in the bottom right hand corner of a map sheet.

 

Reference Ellipsoids

Explanation: 

A reference ellipsoid is an approximation of the Earth's shape. There are many different ellipsoids in use today and each ellipsoid is a "best fit" to the shape of the Earth for a particular region. For example, the International 1924 reference ellipsoid can be used for many areas in Europe. For a more detailed explanation go to the Reference Ellipsoid standards section.

Recommendation: 

The World Geodetic System of 1984 (WGS 84) ellipsoid and its associated datum (WGS 84) is recommended for use since it best approximates the shape of the Earth globally and not just locally.

Alternative: 

It is not always possible to use the WGS 84 ellipsoid and in that case an ellipsoid that best approximates the shape of the Earth for the particular area of interest should be used. The available data may already be in this reference ellipsoid. This information can be found in the spatial reference information section of the data's metadata or in the bottom right hand corner of a map sheet.

 

Coordinate Systems

Explanation: 

A coordinate system is a way to describe points in two or three-dimensional space. In the case of geography, coordinates define a particular place on the Earth. An example of a two dimensional geographic coordinate system is the Universal Transverse Mercator (UTM) system. This system uses only two coordinates to describe a horizontal position on the Earth. An example of a three dimensional geographic coordinate system is the latitude/longitude (lat/long) system. The coordinates of the latitude/longitude system are defined as follows. The latitude is the angular distance of a point on the Earth's surface along a meridian north or south of the equator. The longitude is the angular distance of a point on the Earth's surface east or west of an arbitrarily defined meridian, usually the Greenwich meridian (Greenwich, England) (From AGI On-line GIS Dictionary). This system is three-dimensional because it adds the element of altitude. The altitude is the distance between a defined point and the surface of a reference ellipsoid. The coordinates of a point are specified by the angles lat and long, and the distance from the surface of the ellipsoid, z.

Recommendation: 

The Latitude/Longitude coordinate system and the Universal Transverse Mercator (UTM) coordinate systems are recommended for use. The UTM system is based on the recommended UTM projection and the latitude/longitude system is very widely used.

Several companies offer tools that will perform transformations between coordinate systems. There are two coordinate transformation tools available for purchase from Blue Marble Geographics: the Geographic Calculator and the Geographic Translator. From Mentor Software, Inc. Available for purchase from Linden Software Limited is the DatumPro software that will perform conversions between datums and coordinate systems.

It is not always possible to use the UTM or lat/long coordinate systems and in that case the coordinate system that best fits the area of interest should be used. Usually, this is a local coordinate system and will be in use by the available data. This information can be found in the spatial reference section of the data's metadata or in the bottom right hand corner of a map sheet.

 

Transfer Standard

Explanation: 

Spatial data transfer standards specify ways to transfer spatial data between different hardware systems and software packages through the use of an intermediate exchange file format.

There are several transfer standards in use today. A few of the major ones are the Spatial Data Transfer Standard (SDTS) developed by the USGS, Digital Geographic Information Exchange Standards (DIGEST) which is used in military applications within many NATO countries and the ISO/IEC 8211 developed by the International Standards Organization. The Open GIS Consortium is currently developing a transfer technology model.

Recommendation:

It is recommended that either the ISO/IEC 8211 standard or the transfer technology developed by the Open GIS Consortium should be used when it is complete. Both of these transfer standards are meant for international use not just for a particular country or application.

Alternative:

It will not always be possible to use the recommended standards and in that case other standards such as the Spatial Data Transfer Standard or the Digital Geographic Information Exchange Standards should be used when transferring data. The standard you want to use depends on the data you need to transfer.

 

Metadata

Explanation: 

Metadata is data about data. Metadata consists of information that describes spatial data and is used to provide documentation for data products. For example, metadata describes the format, geographic boundary, projection, coordinate system, availability and cost, etc., of a spatial data set. Currently, there are several efforts to implement standards with regards to metadata. Refer to the Metadata section within Spatial Data Issues for a more detailed explanation.

Recommendation: 

The ISO metadata standard is the recommended standard to use when creating metadata. It appears that other major metadata standards will be either consistent or compatible with this standard and it will be used by the Open GIS Consortium when final standards are published.

 

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