Data Set Characteristics | Data Set Development | Data Sources | Data Processing | Accuracy | GTOPO30 Caveats | Grid Spacing & Resolution | Topographic Detail & Accuracy | Production Artifacts | Summary | References | Disclaimers

Technical Notes for GTOPO30 Terrain Data

GTOPO30 is a global digital elevation model (DEM) resulting from a collaborative effort led by the staff at the U.S. Geological Survey's EROS Data Center in Sioux Falls, South Dakota. Elevations in GTOPO30 are regularly spaced at 30-arc seconds (approximately 1 kilometer). GTOPO30 was developed to meet the needs of the geospatial data user community for regional and continental scale topographic data. This release represents the completion of global coverage of 30-arc second elevation data that have been available from the EROS Data Center beginning in 1993. Several areas have been updated and the entire global data set has been repackaged, so these data supersede the previously released continental data sets.

Data Set Characteristics

GTOPO30 is a global data set covering the full extent of latitude from 90 degrees south to 90 degrees north and the full extent of longitude from 180 degrees west to 180 degrees east. The horizontal grid spacing is 30-arc seconds (0.008333333333333 degrees), resulting in a DEM having dimensions of 21,600 rows and 43,200 columns. The horizontal coordinate system is decimal degrees of latitude and longitude referenced to WGS84. The vertical units represent elevation in meters above mean sea level. The elevation values range from -407 to 8,752 meters. In the DEM, ocean areas have been masked as "no data" and have been assigned a value of -9999. Lowland coastal areas have an elevation of at least 1-meter, so in the event that you reassign the ocean value from -9999 to 0, the land boundary portrayal will be maintained. Due to the nature of the raster structure of the DEM, small islands in the ocean less than approximately 1 square kilometer will not be represented.

Data Set Development

GTOPO30, completed in late 1996, was developed over a 3-year period through a collaborative effort led by staff at the US Geological Survey's EROS Data Center (EDC). The following organizations participated by contributing funding or source data: the National Aeronautics and Space Administration (NASA), the United Nations Environment Programme/Global Resource Information Database (UNEP/GRID), the US Agency for International Development (USAID), the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, the Geographical Survey Institute (GSI) of Japan, Manaaki Whenua Landcare Research of New Zealand, and the Scientific Committee on Antarctic Research (SCAR).

Data Sources

GTOPO30 is based on data derived from 8 sources of elevation information, including vector and raster data sets. The following table lists the percentage of the global land surface area derived from each source (see table):

Data sources

Source	% of global land area
Digital Terrain Elevation Data	50.0
Digital Chart of the World	29.9
USGS 1-degree DEMs	6.7
Army Map Service 1:1,000,000-scale maps	1.1
International Map of the World 1:1,000,000-scale maps	3.7
Peru 1:1,000,000-scale map	0.1
New Zealand DEM	0.2
Antarctic Digital Database	8.3

Data Processing

GTOPO30 was developed over a 3-year period during which continental and regional areas were produced individually. As such, processing techniques were developed and refined throughout the duration of the project. Although the techniques used for the various continental areas are very similar, there were some differences in approach due to varying source material. More details about data development for several of the continental areas are reported by Verdin and Greenlee (1996), Bliss and Olsen (1996), and Gesch and Larson (1996).

Data processing was accomplished using commercially available geographic information system software, public domain image processing software, vector-to-raster gridding software, and utilities developed specifically for this project. To more efficiently handle the numerous input data sets and to standardize the proper sequence of processing steps, the production procedures were automated to a great extent by employing preset parameter values, scripted command files, and consistent naming schemes for input and output data files.

Accuracy

The absolute vertical accuracy of GTOPO30 varies by location according to the source data. Generally, the areas derived from the raster source data have higher accuracy than those derived from the vector source data. The full resolution 3-arc second DTED and USGS DEMs have a vertical accuracy of +/- 30 meters linear error at the 90 percent confidence level (Defense Mapping Agency, 1986; U.S. Geological Survey, 1993). If the error distribution is assumed to be Gaussian with a mean of zero, the statistical standard deviation of the errors is equivalent to the root mean square error (RMSE). Under those assumptions, vertical accuracy expressed as +/- 30 meters linear error at 90 percent can also be described as a RMSE of 18 meters. The areas of GTOPO30 derived from DTED and USGS DEMs retain that same level of accuracy because, through generalization, a representative elevation value derived from the full resolution cells is chosen to represent the area of the reduced resolution cell (although the area on the ground represented by that one elevation value is now much larger than the area covered by one full resolution cell).

The absolute vertical accuracy of the DCW, the vector source with the largest area of coverage, is stated in its product specification as +/- 650 meters linear error at the 90% confidence level (Defense Mapping Agency, 1990). Experience has shown that the grids derived from DCW data should in many areas be much more accurate than the 650-meter specification. To better characterize the accuracy of the areas of GTOPO30 derived from DCW vector hypsography, the DCW grid was compared to 30-arc second DTED, which averaging had aggregated. By aggregating, the comparison could be done at the 30-arc second cell size of the DCW grid. The comparison was done for portions of southern Europe and the Middle East, and all of Africa. Eliminated from the comparison were those areas of the DCW grid for which supplemental DTED point control had been included in the gridding process. If the averaged DTED are thought of as the reference data set, the RMSE of the DCW grid is 95 meters. To get an idea of the overall absolute accuracy of the DCW grid, the relative error between the DCW and DTED can be combined with the known error of the DTED itself in a sum of squares. The root of that sum of squares is 97 meters. Using the assumptions about the error distribution cited above, a RMSE of 97 meters could be expressed as +/- 160 meters linear error at 90 percent confidence. This number compares favorably with an expected vertical accuracy (linear error at 90 percent) of one-half of the primary contour interval of 1,000 feet (305 meters) for the topographic maps on which the DCW is based.

The accuracy of the areas of GTOPO30 based on the other sources can only be estimated based on that which is known about each source. Using certain assumptions, the vertical accuracy of each source (and the derived 30-arc second grid) can be estimated from the contour interval. One assumption is that the original map sources meet the commonly used accuracy standard, which states that 90% of the map elevations are within +/- 1/2 of the contour interval. It is unknown if any of these maps actually meet this standard. Also, map digitizing and elevation surface interpolation errors are unknown and therefore not included. The table below lists the estimated absolute vertical accuracy for the areas of GTOPO30 derived from each source, with the method of estimating the accuracy also identified. The RMSE numbers were calculated using the assumptions about the error distribution cited above (a Gaussian distribution with a mean of zero).

Vertical accuracy in meters

Source	L.E. at 90%	RMSE	Estimation method
DTED	30	18	product specification
DCW	160	97	calculated vs. DTED
USGS DEM	30	18	product specification
AMS maps	250	152	estimated from 500-meter interval
IMW maps	50	30	estimated from 100-meter interval
Peru maps	500	304	estimated from 1,000-meter interval
N.Z. DEM	15	9	estimated from 100-foot interval
ADD	highly variable	wide range of scales and intervals

Local differences among DEM grid cells are often analyzed to calculate slope and other land surface parameters. The relative vertical accuracy (or point-to-point accuracy on the surface of the elevation model), rather than the absolute accuracy, determines the quality of such parameters derived from local differencing operations. Although not specified for this data set, for many areas the relative accuracy is probably better than the estimated absolute accuracy.

GTOPO30 Caveats

As with all digital geospatial data sets, if you use GTOPO30, you must be aware of certain characteristics of the data set (resolution, accuracy, methods of production, and any resulting artifacts, etc.) in order to better judge its suitability for a specific application. A characteristic of GTOPO30 that renders it unsuitable for one application may have no relevance as a limiting factor for its use in a different application. Because only the end user can judge the applicability of the data set, it is the responsibility of the data producer to describe the characteristics of the data as fully as possible, so that you can make an informed decision.

Grid Spacing & Resolution

Latitude and ground distance

Latitude (deg)	Ground distance (meters)
	E/W	N/S
Equator	928	921
10	914	922
20	872	923
30	804	924
40	712	925
50	598	927
60	465	929
70	318	930
73	272	930
78	193	930
82	130	931

The variation in ground dimensions for one 30-arc second cell should be especially considered for any application that measures area of or distance across a group of cells. Derivative products, such as slope maps, drainage basin areas, and stream channel length, will be more reliable if they are calculated from a DEM that has been first projected from geographic coordinates to an equal area projection, so that each cell, regardless of latitude, represents the same ground dimensions and area as every other cell.

You should maintain the distinction between grid spacing and resolution. Even though the global data set has a consistent 30-arc second grid spacing, not all topographic features that one would expect to be resolved at that spacing will be represented. The level of detail of the source data determines whether the 30-arc second sampling interval is truly appropriate for resolving the important topographic features represented in the source. Certainly, a 30-arc second grid spacing is appropriate for the areas derived from higher resolution DEMs (DTED, USGS DEMs, and the New Zealand DEM), and 30-arc seconds has been shown to be suitable as the cell spacing for grids derived from DCW hypsography (Hutchinson, 1996; Shih and Chiu, 1996). However, coverage of DCW contours is not complete, and there are areas for which elevations were interpolated based only on very sparse DCW point data and/or distant contours. Small areas of this nature are located in Africa, South America, and islands of southeast Asia, while Australia and Greenland contain larger such areas. Also, the quality of the contours from the ADD for the interior of Antarctica does not realistically support a 30-arc second (or even 1-kilometer) grid spacing, although such data are provided for completeness and consistency of the global product.

Topographic Detail & Accuracy

Differences in topographic detail among the sources are evident in GTOPO30. This change in level of topographic information is especially evident at the boundary between areas derived from DTED and DCW in regions of higher relief. The mosaicing techniques that were used resulted in a smoothing of the transition areas, but the change in detail between the two sources remains very noticeable. Even if the same topographic feature (ridge, stream, valley, lake, etc.) is represented in the data derived from the two sources, the elevations across the feature may change somewhat abruptly due to the varying accuracy of the sources. Derived products, such as slope maps, for the source transition areas also emphasize the differences in topographic information derived from the varying sources.

You are reminded that the accuracy levels described above are estimates, and that the accuracy for specific locations within the overall area derived from any one source can vary from the estimate. For instance, approximately 30% of the DTED 1-degree by 1-degree tiles (the production and distribution unit for full resolution DTED) have an absolute vertical accuracy worse than the product specification of +/- 30 meters at 90% confidence. Also, the actual accuracy for some areas derived from the vector contour sources may be better or worse than the estimate. When the map source had multiple contour intervals, the largest interval was used for a conservative estimate. In contrast, some areas may be worse than the estimate because no contour coverage was available for those specific locations.

Production Artifacts

Artifacts due to the production method are apparent in some areas of GTOPO30. While the magnitude of the artifacts in a local area are usually well within the estimated accuracy for the source, you are nonetheless made aware because the effects are plainly visible and they may affect some applications of the DEM. Some areas derived from DTED, especially in Africa and the Middle East, exhibit a striping artifact, most likely due to the production method of the DTED. The artifact is very evident in the full resolution data, but remains noticeable even in the generalized 30-arc second version. Generally, the pattern is more noticeable in low relief areas, while in higher relief areas it is masked by the actual terrain variation. Another pattern seen in some areas derived from DTED is a blocky appearance, which is a reflection of the 1-degree tiling structure of the full resolution DTED. These areas derived from contiguous DTED 1-degree tiles appear blocky because of vertical offsets among the tiles in the original full resolution DTED. The artifacts in the DTED areas may or may not be visible, depending on the method used to display the data. For instance, when viewing the DEM data as an image, either in shades of gray or color, the artifacts may be hidden, depending on the number of shades or colors used. If the data are displayed as a shaded relief image, the appearance of the artifacts will vary depending on the direction of illumination, vertical exaggeration applied, and the scale of the display. Generally, none of the artifacts will be visible on a small-scale portrayal of the global data set.

Some production artifacts are also present in the areas derived from the vector sources. Small artificial mounds and depressions may be present in localized areas, particularly where steep topography is adjacent to relatively level areas, and the hypsography data were sparse. Additionally, a "stair step" (or terracing) effect may be seen in profiles of some areas, where the transition between contour line elevations does not slope constantly across the area but instead is covered by a flat area with sharper changes in slope at the locations of the contour lines. When a histogram of elevations is presented, there are sharp peaks at elevations that are multiples of the contour intervals of the source. This effect is common in DEMs produced by gridding of contour data in which the interpolation process favors elevations at or near the contour values, thus leading to a greater frequency of those elevations. Every effort to reduce these effects has been made by careful selection of parameters for the interpolation process, but some level of these conditions inevitably remain due to the nature of vector-to-raster surface generation.

Summary

GTOPO30 provides a new level of detail in global topographic data. Previously, the best available global DEM was the ETOPO5 data set and its successor TerrainBase, with a horizontal grid spacing of 5-arc minutes (approximately 10 kilometers) (Row, Hastings, and Dunbar, 1995). GTOPO30 data are suitable for many regional and continental applications, such as climate modeling, continental-scale land cover mapping, extraction of drainage features for hydrologic modeling (Danielson, 1996; Verdin and Greenlee, 1996), and geometric and atmospheric correction of medium and coarse resolution satellite image data (Gesch, 1994; Jet Propulsion Laboratory, 1997).

Disclaimers

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Please note that some U.S. Geological Survey (USGS) information contained in this data set and documentation may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are sole responsibility of the user.