5.A.i.a. Source Characteristics

DTED Level 1 is produced in 3 arc-second gridding, on a latitude-longitude projection. DTED Level 1 data files have the following characteristics:

• They cover 1^o by 1^o of latitude-longitude, and are called "DTED cells."
• DTED files have their origin in the southwestern corner of the DTED cell. The grid cell registration is centered on that corner. Thus the first 3" DTED grid cell in the file continuing north from 40^oN latitude and east from 40^oE longitude would be centered at exactly that location.
• Grid cell sequencing begins at the southwestern corner (the origin of the file just noted), then continues upwards (column-wise) to the northwestern corner. Successive eastward columns are presented. The final value in the file is the value of the northeastern corner.
• DTED files have 1201 columns by 1201 rows, overlapping at their edge columns and rows with neighboring DTED cells.

DTED Level 1 data are created from a variety of “best-available” sources. These may be:

• cartographic: maps containing elevation contours and/or point values created by national, local, or non-U.S. agencies, or by NIMA itself or its predecessors
• imagery: aerial photography or satellite imagery, from U.S. or non-U.S. sources
• other possible sources such as Global Positioning Survey measurements or satellite altimetry

Sources are not publicly cited. However, certain characteristics of resultant DEMs help interpret general source characteristics. For example, analysis of shaded-relief images digitally derived from DEMs often detect blockiness, striping, or other patterns symptomatic of various data development techniques. Some of these are discussed below, and explicitly illustrated on the GLOBE Web site’s public Beta Test area (linked to the GLOBE Home Page.)

Common artifacts in DTED Level 1 data include:

• Vertical offsets between many 1^o by 1^o DTED files. These may be seen as vertically offset blocks in shaded-relief or slope maps derived from the data. These offsets may be caused by discontinuities in cartographic source materials, such as topographic contour lines that fail to match across map boundaries. They may also be caused by inadequately documented geodetic models, such as incorrect or missing information on projection or datum. Edge "feathering" as an attempt to provide continuity between 1^o by 1^o DTED cells also contributed artifacts of its own.
• Directional biases in resolution. These may be seen as striped patterns in shaded-relief, slope, or aspect maps derived from such data. Such anisotropies in DEMs may be caused by stereoprofiling techniques (often involving oversampling between stereoprofiles to make the final grid). These techniques were sometimes used to create DEMs from analog stereoscopic imagery or undeterminable satellite source sensor characteristics.

The artifacts are common to other DEMs derived by similar techniques from similar sources.

Prototype correction techniques have been developed at the Jet Propulsion Laboratory, Pasadena, California. Presentations at GLOBE Task Team meetings (Nevin Bryant, California Institute of Technology, 1995, verbal communication; see also Ritter and Bryant, 1997) have demonstrated the contribution that such techniques might make toward rectifying some artifacts in DTED. However, in the absence of a more substantial study, the GLOBE Task Team decided not to attempt such repairs for GLOBE Version 1.0.

Between 50^o North and South latitudes, DTED Level 1 data are 3x3 arc-second grids. Outside these latitudes the longitudinal gridding gets coarser:

Stated accuracy objectives are within 50m horizontal (at 90% circular error; twice the standard deviation), and 30m vertical (at 90% linear error; two sigma). However, areas lacking source materials that meet these standards may still have DTED created for them, if adequate alternatives are not envisaged.

DTED seeks to use World Geodetic System 84 (WGS84) for horizontal reference, and Mean Sea Level as vertical reference. However, this may not always be the case as cartographic sources may not be completely or accurately described, and materials from such sources may not be accurately converted to WGS84 and Mean Sea Level.