TerrainBase Global Terrain Model Summary Documentation
- Description
- Data Set Name: National Geophysical Data Center
- TerrainBase Global DTM Version 1.0
- File Name: TBASE.BIN
- Coverage
- Area Coverage: Worldwide
- Coordinate Coverage: 90N to 90S x 180W to 180E
- Topography/bathymetry/both: Both
- Grid Structure
- Cell Dimensions: Latitude: 05'00” Longitude: 05'00”
- Cell Registration: Center of cell
- Total Grid Rows: 2160
- Total Grid Columns: 4320
- Data Characteristics
- Elevation Units: Meters
- Elevation Type:
- Projection: Latitude/longitude (no projection)
- Vertical Accuracy:
- Horizontal Accuracy:
- Null Land Value Designator: N/A
- Null Ocean Value Designator: 0
- Source
- Data Developer:
- Lee W. Row, III and David Hastings
- National Geophysical Data Center and World
- Data Center-A for Solid Earth Geophysics
- Boulder, Colorado U.S.A.
Model Development
Overview
The TerrainBase global digital terrain model contains a complete
matrix of land elevation and ocean depth values for the entire world
gridded at 5-minute intervals. NGDC/WDC-A developed the model using
the best public domain data available at the time of publication.
This version of TerrainBase, called “beta test release 1.0,” is the
first of an ongoing sequence of global terrain models to be produced
and disseminated by NGDC/WDC-A. Subsequent releases will contain
higher quality data and will supersede all previous versions. The
frequency of updates and level of quality enhancements will be
dependent upon the contribution of new source data.
The global model was developed by integrating all of the data sets
described in the previous chapter into a single model. Model
development was a three-step process that entailed regridding each
individual regional model to 5-minutes, then patching each regridded
model together into a single global model, and, finally, smoothing a
few significant discontinuities at the boundaries of adjoining models.
Each of these steps is discussed in detail in the following sections.
The GRASS (version 4.1) geographic information system was used for all
data processing.
A term to describe the type of height values represented in the model
(such as mean, mode, point, etc.) cannot be assigned to the
TerrainBase model since it is comprised of a variety of source data.
The type of terrain height represented by these source data sets
varies from one model to the next. For example, the FNOC model has
modal heights, while the USA model uses point heights, and the Europe
model has mean heights. Consequently, it is not possible to assign a
single term to represent all of the height values given in the
TerrainBase model.
To satisfy interested users, however, a term that can be used which is
common to all of the source data is best estimate of height. That is,
the data represent the best estimate of height for each 5-minute cell.
Although such a term has minimal scientific value, it does emphasize
the fact that the data only represent reasonable estimates of the
height of the terrain.
Regional Model Regridding
The first step in model development entailed regridding each regional
model into 5-minute grids. Twenty-six regional and worldwide terrain
models were used to create the TerrainBase model. Each of these input
models is described individually in detail in the previous chapter. A
5-minute grid spacing was chosen for this model since this is the
highest global resolution that can be supported by the individual data
sets.
Two types of processes were used to regrid the source files. For
source models that had grid spacing smaller than 5-minutes, a matrix
filter was used to average adjoining cells into 5-minute cells.
Averaging entailed taking the non-weighted mean of all input cells that
fell within the boundaries of the 5-minute output cell. For some
models, this process entailed point sampling the data to a smaller grid
cell spacing that is an even divisor of 5-arc-minutes and then
averaging these cells to create 5-minute cells. For source models that
had a grid spacing larger than 5-minutes, a linear interpolation
process was used to regrid the data to 5-minutes.
Averaging with a matrix filter was also used to convert 5-minute models
from a grid intersection registration to a center of cell registration.
Users should note that models that have been re-registered in this
manner are somewhat smoothed in comparison to the original model. These
include only the Australia, and Northwest Territories models.
Each of the individual input models is listed in Figure 5.1 of the
TerrainBase User Manual along with the type of processing that was
used in regridding to 5 minutes.
Data Integration
After each regional model was regridded to 5 minutes, it was integrated
with the other DTMs into a single file of continuous global land and
ocean coverage. The models were combined using a technique in GRASS
called “patching” which permits the integration of gridded files with
arbitrary spatial coverage. This technique generates an output model by
assembling multiple input models in a prioritized order. The highest
quality input models were assigned the highest priority, and the poorest
quality models were assigned the lowest priority. Assembly takes place
by using the highest priority input model and filling in areas of no
data coverage with data from the model that is second in order. Areas
of no data coverage are denoted by cells containing a numeric zero value.
Next the third model is applied to fill in any areas not covered by the
first and second model. This process continues until all models have
been applied in their specified order. The advantage of this process is
that it enables the highest quality input models to cover as much area
of the output model as possible and using the lowest quality models only
to fill in gaps where better quality data is not available. A example
of this scheme is shown in the TerrainBase User Manual.
The order in which the input models were integrated is provided in the
list below. Coverage of input files as they exist in the final global
model are shown in Figures 5.2 through 5.9 of the user manual. U.S.
coastal bathymetric data was not used in the global model due to
substantial offsets between it and the ETOPO5 bathymetry data.
Ordering of Source Data used to Compile the
TerrainBase Global Model
1. Italy |
10. America |
2. Austria |
11. Northwest Territories, Canada |
3. Netherlands |
12. Andes Mountains and |
4. Madagascar |
Peru-Chile Trench |
5. Haiti |
13. Brazil Cerrados |
6. U.S.A. |
14. Australia |
7. Greenland |
15. Japan |
8. Africa |
16. ETOPO5 bathymetry |
9. Europe |
17. FNOC topography |
The next step in model development was testing for significant errors
and artifacts. A number of data spikes were identified in this process
and methods were developed for correcting them. Several dozen spikes
were corrected using an automated method that replaced each erroneous
cell with the mean of the neighboring eight cells. Several other spikes
were corrected by modifying obvious errors such as replacing missing
negative signs or replacing missing zeros. Contact NGDC for a listing
of corrected cells.
Reducing Boundary Discontinuities
One artifact of producing a global model using the method described
above is vertical misalignment between the borders of regional models.
Vertical offsets which break the continuity of the terrain surface
represented by the model have been detected along many of the
boundaries where regional models adjoin. Fortunately, nearly all of
the offsets are small and are well within the vertical tolerance of
the 5-minute global model. The only region where the offsets are
significant are in South America along the boundaries of the Brazil
Cerrados DTM and in Antarctica at the southern edge of the ETOPO5
bathymetry model. Offsets along these boundaries range from tens to
several hundreds of meters.
In South America, the main cause of the offsets is the lower data
accuracy in the areas where the input models adjoin. Offsets are most
prominent in the Amazon Basin, Paraguay, and Southern Brazil, where
the quality of the source models appears relatively poor. In these
areas, the density of source data used to generate the regional models
are low, and consequently the quality of the models in these areas is
poor. The low density of source data manifests itself as areas of low
grid detail in the FNOC model and as broad undulating spurious features
in the Brazil model. Where the two models adjoin, the data show
practically no agreement between each model. Since there is a lack of
good comparative data for the area, it is not yet possible to determine
which model (FNOC, Andes, or Brazil) is more accurate along the
boundaries.
A few different methods were tested to reduce the discontinuities
between data sets in South America. The only method that worked
satisfactorily was to smooth the data along the boundaries using a
matrix filter. A 5x5 filter was applied to an area that extended 5 cell
widths from each boundary. The boundaries where the filter was applied
are shown in Figure 5.10. The filter has the effect of replacing each
cell with the non-weighted mean of itself and the surrounding 24
neighboring cells to create a highly smoothed output grid. Figure 5.11
shows the before and after appearance of a portion of the smoothed
boundary which illustrates the abrupt offset being converted to a
smoothly tapered offset.
Such a modification is merely cosmetic, since filtering does not
improve the reliability of the data. Boundary offsets are undesirable
from a scientific standpoint whether they are smoothed or not. The
only desirable option for correcting these offsets is to replace the
data along the boundaries with more accurate values. Unfortunately,
such a correction is not possible at this time due to the lack of
reliable data for the area. Hopefully, better quality data can be
obtained to alleviate this problem in future releases of TerrainBase.
Discontinuities near Antarctica were not smoothed due to the nature of
the offsets in this area. Below 78 S, where the ETOPO5 bathymetry model
terminates, there are no other bathymetric data available. Ocean cells
south of 78 S were filled with numeric zeros which denote null values.
This produces a broad, flat, zero-elevation terrace extending from 78 S
to the Antarctic coast. At 78 S, a sharp drop-off occurs where the shelf
meets the edge of the bathymetry model. Smoothing this boundary would
have little purpose since it would entail modifying the bathymetry data,
in part, to conform with the null data. In addition, smoothing would
also have minimal impact on the cosmetic appearance of this
discontinuity. This problem only affects the southernmost coastal areas
of the Ross and Weddell Seas.
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