Ancillary Product Documentation: TOPO
Description: TOPO is a fixed data product providing a land-water mask together with
topographic height information over land. A 0.1 degree (“10-km”) version is used in processing
ISCCP L2 (pixel-level) and is reported in the two ISCCP L2 products for each pixel present;
some information from a 1.0 degree version is also used in the cloud analysis. A 1.0 degree
version is reported in the ISCCP L3 (gridded) products. The 0.1 degree version is used in the
SRB L2 processing and the 1.0 degree version is reported in the SRB gridded product. The 1.0
degree version is reported in the GPCP gridded product. The 0.25 degree version is used in the
SeaFlux (land-water mask only) and LandFlux L2 processing and reported in their gridded
products. The 1.0 version is reported in the GEWEX Joint Energy-Water Cycle data product. All
of these versions are constructed directly from the same 0.01 degree basic product.
Format and Contents: The format is netCDF4.0. The product reports land fraction in percent,
“shore” distance in km, the mean and standard deviation of topographic height in meters (only
land points included in the statistics in mixed land-water grid cells). Shore distance is the
distance to the center of the nearest land-dominated (land fraction >65%) or mixed land-water
(coast) grid cell if the map grid cell is water-dominated (land fraction < 35%) or to the nearest
water-dominated or coast grid cell if the map grid cell is land-dominated. The distance is 0 km
for coast map grid cells. Versions are available in equal-area map grids, equivalent at the equator
to latitude-longitude intervals of 0.1, 0.25, 0.50, 1.0, 2.0 and 2.5 degrees, all produced directly
from the same 0.01 degree version.
Quantities Reported in Map Grid Cell
Quantity
Units
Valid Range Fill Value
Land fraction
Percent0 to 100
32767
Mean topographic height above mean sea level
Meters -405 to 6626 32767
Standard deviation of topographic height
Meters 0 to 1351
32767
Distance to land for all water cells
Kilometers
0 to 11500
32767
Distance to water for all land cells
Kilometers
0 to 11500
32767
Input Data Sources: One land-water mask is from the USGS Global Land 1-KM AVHRR Project
obtained from the USGS EROS data center at:
http://edc2.usgs.gov/1KM/land_sea_mask.php.
and compiled from the World Vector Shoreline data set for ocean boundaries and the Digital
Chart of the World for the large inland lakes, estuaries and rivers. These sources were rasterized
to 1-km grid cells, where the type is assigned based on dominant area coverage. The original
mapping is a Lambert Azimuthal Equal Area projection.
The global digital elevation model used to provide land topographic heights is
GTOPO30, obtained from the USGS EROS data center at:
http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/gtopo30_info
Topographic height in meters above mean sea level is reported at intervals of 30-arc seconds
(0.008333 degrees) referenced to WGS84. Ocean areas have been masked to a “no data” value of
−9999, hence this product also provides a land-water mask. The Digital Elevation Model is
compiled from eight sources. Complete documentation can be found at:
http://http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/gtopo30_info
and
http://http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/gtopo30/README
The original product mapping is the Interrupted Goode Homolosine projection broken up into
smaller tiles for convenience.
Comparisons to Other Products/Evaluation: GTOPO30 was compared to ETOPO2v2 and the
SRTM products. ETOPO2v2 is lower resolution than GTOPO30 but includes bathymetry for
oceans and large lakes. Ignoring the differences for inland lakes, the main differences (less than
± 200 m) appear over high terrain, where resolution effects would be expected. However, there
are larger differences up to 500 m over Greenland, both high and low, as well as along the
southern boundary of the Himalayas. GTOPO30 was selected for use because it is a global
digital elevation model at smaller spatial intervals and makes a clear distinction between
continents and oceans. The land-water masks in the GTOPO30 product was compared to the 1KM AVHRR Project product; differences were reconciled as discussed below.
Processing Steps:
A. Modifications to Original: The 1-KM AVHRR Project product was re-mapped into the
ISCCP-style equal-area grid at the same effective resolution using a nearest neighbor resampling approach and inspected for any flaws (missing values) produced by this step (none
found). The GTOPO30 data were re-mapped to the same 1-km equal-area grid. In this step, the
land fraction and (mean) topographic height are calculated.
B. Filling Methods: None required.
C. Reconciliation: The two land-water masks are reconciled in the ISCCP 1 km grid in favor of
the GTOPO30 land fraction as follows:
GTOPO30 Land Fraction // AVHRR Land Fraction
0
> 0 but < 100
100
0
100
0
GTOPO value
0
0
GTOPO value
100
The replacement of the GTOPO30 land fraction in the interior of continents when the 1-KM
AVHRR reports all water is necessary to identify large lakes but the GTOPO30 topographic
information is retained for these locations so they can be distinguished from open ocean
locations which have no topographic information. During this processing an ice shelf fraction is
determined for all locations poleward of 60 degrees where the AVHRR land fraction = 100% but
GTOPO30 gives < 100%: in this case the ice shelf fraction is given by by 100 – GTOPO30 land
fraction and land fraction is set to 0%. This dataset is further processed in creating the SRFTYP
product.
D. Merging and Derivation of Additional Quantities: The GTOPO30 dataset at 1-km, after the
modifications to reconcile with the AVHRR Project land-water mask, is used to produce all other
versions of the ISCCP TOPO product on equal-area map grids, equivalent to 0.1, 0.25, 0.50, 1.0,
2.0 and 2.5 degrees at the equator. All lower resolution values of land fraction, mean topographic
height and standard deviation of topographic height are calculated directly from the 1-km
product. In each mapping, an additional quantity, called “shore”, is determined as follows. First,
each map grid cell is classified as water (land fraction < 35%), land (land fraction > 65%) or
coast (intermediate land fractions). Then for grid cells classified as water, the distance from the
center of the grid cell to the center of the nearest grid cell classified as either land or coast, up to
115 km, is determined. For grid cells classified as land, the distance to the center of the nearest
grid cell classified as either water or coast is determined. For grid cells classified as coast, the
shore distance is set to zero.