Masks and Overlays
The masks and overlays provided here supply the user with many options for concealing unwanted regions or overlaying coastlines and latitude/longitude grids; they are divided into the following categories:
Land Masks
The land masks are useful for concealing the land of the Northern and Southern Hemispheres prior to processing of the data. The files are divided into two categories: Northern Hemisphere and Southern Hemisphere.
Note on Land Masks
Several versions of the land masks have been developed, though NSIDC only distrubutes the most current masks.
Beginning with the SMMR era, the first masks NSIDC developed were landmask.ntb and landmask.stb. Other masks were later added for use with SSM/I F8 data. The last set of masks to be added to the series are those beginning with gsfc, which were originally developed for use with SSM/I F11 and F13. However, all of the masks provided here will work across all platforms (SMMR and SSM/I).
The relatively slight differences in numbers of SSM/I-grid pixels masked as land can introduce discrepancies in analyses of time series spanning the SMMR and SSM/I period. One method of addressing this issue is to generate a composite mask in which all pixels mapped as land in any of the masks are coded as land pixels in the composite mask. Use of such a composite mask improves the consistency of the SMMR and SSM/I record at the expense of masking additional ocean areas as land. A composite of the CIA World Data Bank I and II (GSFC-1 and JPL-1) has been produced by J. Maslanik at NSIDC.
Another issue is the effect of contamination of coastal ocean pixels by their proximity to land. Such proximity can modify the brightness temperatures of coastal ocean pixels, producing false ice concentration values along some coasts. These pixel mixing errors are considered in the summer 1996 issue of NSIDC Notes (Issue no. 18). Maslanik et al. (1996) discuss the effects of such land contamination in introducing differences between SMMR and SSM/I time series, and they describe the use of a modified land mask where land areas are extended to mask substantial contamination. The Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I Brightness Temperatures land mask (n3blcmsk.hdf) alleviates this problem whereby ocean pixels adjacent to land or one pixel removed from land are designated as land-contaminated.
The land masks that are availabe are the following:
- amsr_gsfc_6n.hdf and amsr_gsfc_6s.hdf
- amsr_gsfc_12n.hdf and amsr_gsfc_12s.hdf
- amsr_gsfc_25n.hdf and amsr_gsfc_25s.hdf
- gsfc_12n.msk and gsfc_12s.msk
- gsfc_25n.msk and gsfc_25s.msk
- landmask.ntb and landmask.stb
- n3blcmsk.hdf
- pole_n.msk
| Tool Name | amsr_gsfc_6n.hdf and amsr_gsfc_6s.hdf |
|---|---|
| Applicable Data Sets | AMSR-E/Aqua Daily L3 6.25 km 89 GHz Brightness Temperature Polar Grids |
| Description | 6.25 km AMSR-E land mask for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). Although the AMSR-E land masks are more current than those NSIDC uses for SSM/I data, they have not been updated since 1997 to account for many iceberg calvings. A 1-byte integer array is included in each HDF file. Values are 0 (water), 3 (coast), and 216 (land). amsr_gsfc_6n.hdf:1216 columns x 1792 rows amsr_gsfc_6s.hdf: 1264 columns x 1328 rows |
| Access | FTP |
| Tool Name | amsr_gsfc_12n.hdf and amsr_gsfc_12s.hdf |
|---|---|
| Applicable Data Sets | AMSR-E/Aqua Daily L3 12.5 km Brightness Temperature, Sea Ice Concentration, & Snow Depth Polar Grids |
| Description | 12.5 km AMSR-E land mask for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). The North 12.5 km AMSR-E land mask is identical to gsfc_12n.msk, while the South land mask is based on data from the 1997 RADARSAT Antarctic campaign. Although the AMSR-E land masks are more current than those NSIDC uses for SSM/I data, they have not been updated since 1997 to account for many iceberg calvings. A 1-byte integer array is included in each HDF file. Values are 0 or 1, where 1 is the mask. amsr_gsfc_12n.hdf: 608 columns x 896 rows amsr_gsfc_12s.hdf: 632 columns x 664 rows |
| Access | FTP |
| Tool Name | amsr_gsfc_25n.hdf and amsr_gsfc_25s.hdf |
|---|---|
| Applicable Data Sets | AMSR-E/Aqua Daily L3 25 km Brightness Temperature & Sea Ice Concentration Polar Grids |
| Description | 25 km AMSR-E land mask for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). The North 25 km AMSR-E land mask is identical to gsfc_25n.msk, while the South land masks are based on data from the 1997 RADARSAT Antarctic campaign. Although the AMSR-E land masks are more current than those NSIDC uses for SSM/I data, they have not been updated since 1997 to account for many iceberg calvings. A 1-byte integer array is included in each HDF file. Values are 0 or 1, where 1 is the mask. amsr_gsfc_25n.hdf: 304 columns x 448 rows amsr_gsfc_25s.hdf: 316 columns x 332 rows |
| Access | FTP |
| Tool Name | gsfc_12n.msk and gsfc_12s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures
Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures |
| Description | 12.5 km land and coast mask for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). A 1-byte integer array is included in each file. Values are 0 or 1, where 1 is the mask. gsfc_12n.msk: 608 columns x 896 rows gsfc_12s.msk: 632 columns x 664 rows |
| Access | FTP |
| Tool Name | gsfc_25n.msk and gsfc_25s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data Nimbus-5 ESMR Polar Gridded Brightness Temperatures Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations |
| Description | 25 km land and coast mask for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). A 1-byte integer array is included in each file. Values are 0 or 1, where 1 is the mask. gsfc_25n.msk: 304 columns x 448 rows gsfc_25s.msk: 316 columns x 332 rows |
| Access | Brightness Temperature Data Sets:
FTP Sea Ice Data Sets: FTP |
| Tool Name | landmask.ntb and landmask.stb |
|---|---|
| Applicable Data Sets | Nimbus-5 ESMR Polar Gridded Brightness Temperatures |
| Description | Northern (ntb) and Southern (stb) Hemisphere land mask at a 25 km resolution. Note: These have been superseded by the gsfc_25n.msk and gsfc_25s.msk but may be more appropriate for the ESMR data sets.
landmask.ntb: 304 columns x 448 rows landmask.stb: 316 columns x 332 rows |
| Access | FTP |
| Tool Name | n3blcmsk.hdf |
|---|---|
| Applicable Data Sets | Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I Brightness Temperatures |
| Description | Northernth Hemisphere, 25 km land mask. Ocean pixels adjacent to land or one pixel removed from land are designated as land-contaminated.
304 columns x 448 rows |
| Access | FTP |
| Tool Name | pole_n.msk |
|---|---|
| Applicable Data Sets | Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data|
| Description | Circular mask that symmetrically covers the observed maximum extent of the missing data (resulting from the orbit inclination and instrument swath) near the North Pole. This is a 1-byte array with no header. The area of the SSM/I hole has a value of 1. The additional area of the SMMR hole (a ring around the SSM/I hole) has a value of 2. All other pixels have a value of 0. pole_n.msk: 304 columns x 448 rows |
| Access | FTP |
Land Overlays
The land overlays are useful for overlaying coast line and latitude/longitude grids on top of images and processed data. The files are broken down into two categories: Northern Hemisphere and Southern Hemisphere.
The land overlays that are availabe are the following:
- coast_12n.msk and coast_12s.msk
- coast_25n.msk and coast_25s.msk
- ltln_12n.msk and ltln_12s.msk
- ltln_25n.msk and ltln_25s.msk
| Tool Name | coast_12n.msk and coast_12s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures
Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures |
Description | 12.5 km coastline grid overlay for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). Format is 1-byte integer arrays. Values are 0 or 1, where 1 is the mask. coast_12n.msk: 608 columns x 896 rows coast_12s.msk: 632 columns x 664 rows |
| Access | FTP |
| Tool Name | coast_25n.msk and coast_25s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data Nimbus-5 ESMR Polar Gridded Brightness Temperatures Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations |
| Description | 25 km coastline grid overlay for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). Format is 1-byte integer arrays. Values are 0 or 1, where 1 is the mask. coast_25n.msk: 304 columns x 448 rows coast_25s.msk: 316 columns x 332 rows |
| Access | Brightness Temperature Data Sets:
FTP Sea Ice Data Sets: FTP |
| Tool Name | ltln_12n.msk and ltln_12s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures
Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures |
| Description | 12.5 km latitude/longitude grid overlay for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). Format is 1-byte integer arrays. Values are 0 or 1, where 1 is the mask. ltln_12n.msk: 608 columns x 896 rows ltln_12s.msk: 632 columns x 664 rows |
| Access | FTP |
| Tool Name | ltln_25n.msk and ltln_25s.msk |
|---|---|
| Applicable Data Sets | DMSP SSM/I Daily Polar Gridded Brightness Temperatures Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data Nimbus-5 ESMR Polar Gridded Brightness Temperatures Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Near Real-Time DMSP SSM/I Daily Polar Gridded Brightness Temperatures Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations |
| Description | 25 km latitude/longitude grid overlay for both hemispheres (n: Northern Hemisphere, s: Southern Hemisphere). Format is 1-byte integer arrays. Values are 0 or 1, where 1 is the mask. ltln_25n.msk: 304 columns x 448 rows ltln_25s.msk: 316 columns x 332 rows |
| Access | Brightness Temperature Data Sets:
FTP Sea Ice Data Sets: FTP |
Missing Data Mask
The circular section over the Northern Hemisphere pole (known as the
pole hole) is never measured due to orbit inclination. The following
mask is available for the pole hole region not covered by the SMMR
and SSM/I instruments.
| Tool Name | pole_n.msk |
|---|---|
| Applicable Data Sets | Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data|
| Description | Circular mask that symmetrically covers the observed maximum extent of the missing data (resulting from the orbit inclination and instrument swath) near the North Pole. This is a 1-byte array with no header. The area of the SSM/I hole has a value of 1. The additional area of the SMMR hole (a ring around the SSM/I hole) has a value of 2. All other pixels have a value of 0. pole_n.msk: 304 columns x 448 rows |
| Access | FTP |
Region Masks
The region masks are smaller areas than the land masks although they
can be used as land masks using the pixel value of 11.
| Tool Name | region_n.msk and region_s.msk | ||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Applicable Data Sets | Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data|||||||||||||||||||||||||||||||||||||||||||
| Description | Region masks for the Northern (n) and Southern (s) Hemisperes. The files contain a 300-byte header, followed by a two-dimensional byte array stored by rows in column order. Regions are assigned different pixel values as shown in the following table. These region masks are described further in "Arctic and Antarctic Sea Ice, 1978-1987 Satellite Passive Microwave Observations and Analysis," NASA SP-511. region_n.msk: 448 rows x 304 columns region_s.msk: 316 rows x 332 columns Regions are assigned different byte values as follows:
|
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| Access | FTP |
Ocean Masks
| Tool Name | For a full list of ocean masks and maximum extent masks, refer to the Monthly Ocean Masks and Maximum Extent Masks Web page. |
|---|---|
| Applicable Data Sets | Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data|
| Description | Monthly Ocean Masks and Maximum Extent Masks are available for the Northern (n) and Southern (s) Hemispheres. For more information and a description of the files, refer to the Monthly Ocean Masks and Maximum Extent Masks Web page. |
| Access | See the Data Access section of the Monthly Ocean Masks and Maximum Extent Masks Web page for FTP access. |
