GLOBE Review Paper

The Global Land One-km Base Elevation (GLOBE) Project
Candidate Review: DEM for Italy Public Comments are Solicited

The GLOBE project has three digital elevation models (DEMs) for Italy to choose from for the final 1-km global digital elevation model. The three candidate DEMs were contributed by the Servizio Geolgico Nazionale Italy (SGN), the U.S. Geological Survey EROS Data Center (EDC), and the National Imagery and Mapping Agency (NIMA). The results of the evaluation of the three DEMs are found below.

The first section includes a table showing when each DEM equaled the minimum, maximum, and median of the three DEMs.

The second section displays histograms created from each digital elevation model.

In the third section, two selected areas of Italy (the island of Sicily and northern Italy) are displayed as color palette images of the digital elevation data, shaded relief images, and aspect maps.

We are asking for input from the GLOBE partners and anyone else who is interested in analyzing the results we have obtained from the three models so far. Although review for GLOBE Version 1.0 is essentially complete, comments are still encouraged to help design future enhancements to GLOBE. Please telephone or email David Hastings with any comments.

A table listing the results of an evaluation of when each DEM equaled the minimum, maximum, and median of all three DEMs is listed below. It can be seen from this table that the median values (255869 hits) occur most often in the EDC DEM, the maximum values (275925 hits) occur most often found in the SGN DEM,and the minimum values (384366 hits) occur most often in the NIMA DEM. In addition, the same values occur most often in the NIMA and EDC DEMs (104692 hits) and least often in the NIMA and SGN DEMs (6576 hits). The similarity of the NIMA and EDC DEMs is not surprising, since these DEMs were derived from the same source materials (3-arc-second DTED data).

To view histograms from the three DEMs for Italy click on the icons listed below.

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

DISCUSSION

Spikes at 100 m, 200 m, 300 m ... 1000 m, are evident in the histograms from the EDC and NIMA DEMs. This terracing effect, where certain height values occur in a model much more frequently than other height values, often results when terrain models are derived from topographic maps. The large peaks often correspond to contour intervals on the source maps. The source data for both of these DEMs was the former Defense Mapping Agency (DMA) Digital Terrain Elevation Data (DTED). DTED are developed on a 3-arc-second latitude longitude grid, showing elevations above sea level based on the World Geodetic System 84 datum. Source materials for DTED may include digital elevation models, sterographic imagery, or topographic maps. The terracing effect displayed in both histograms indicates the source materials for these DTED data were probably topographic maps. The differences in the two DEMs are due to differences in how the DTED data were resampled to produce the final 30-arc-second DEMs.

Contrasted with the EDC and NIMA histograms, the histogram for the SGN DEM shows a very smooth curve with no peaks. The source for this DEM was a 7.5 x 10-second DTM developed by the Italian Geological Service for use in gravity terrain corrections.

To view color palette images for the island of Sicily and northern Italy, from the three Italian DEMs click on the icons listed below.

Color palette images for the island of Sicily:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

Color palette images for northern Italy:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

To view shaded relief images for the island of Sicily and northern Italy, from the three Italian DEMs click on the icons listed below.

Shaded relief images for the island of Sicily:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

Shaded relief images for northern Italy:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

DISCUSSION

The level of detail as displayed in the shaded relief images is very similar in all three DEMs.

To view aspect maps for the island of Sicily and northern Italy, from the three Italian DEMs click on the icons listed below.

Aspect maps for the island of Sicily:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

Aspect maps for northern Italy:

• Servizio Geologico Nazionale Italy
• U.S. Geological Survey EROS Data Center (EDC)
• National Imagery and Mapping Agency (NIMA)

DISCUSSION

The level of detail as displayed in the aspect maps is very similar in all three DEMs. Although artifacts can be found in the three images when rigorously compared, the quality of the three DEMs appears to be very similar.