This data set (NmIcEdg2) estimates the location of the North and South Pole sea ice edges at various times during the mid to late 1960s, based on recovered Nimbus 1 (1964), Nimbus 2 (1966), and Nimbus 3 (1969) visible imagery.
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Nimbus Ice Edge Points from Nimbus Visible Imagery L2, CSV, Version 1
|Temporal Resolution:||7 day to 8 day|
|Platform(s)||NIMBUS-1, NIMBUS-2, NIMBUS-3|
|Data Contributor(s):||David Gallaher, G. Garrett Campbell|
|Metadata XML:||View Metadata Record|
As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.Gallaher, D. and G. Campbell. 2014. Nimbus Ice Edge Points from Nimbus Visible Imagery L2, CSV, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/NIMBUS/NmIcEdg2. [Date Accessed].
Detailed Data Description
This data set consists of geographical points (longitude, latitude) that indicate the likely location of the North and South Pole sea ice edges at various times during the mid to late 1960s, based on recovered visible imagery from the Nimbus 1, Nimbus 2, and Nimbus 3 satellite missions. The ice edge was located by manually inspecting thousands of images from the Nimbus Advanced Vidicon Camera System Visible Imagery L1, HDF5 (before 1968) and Image Dissector Camera System Visible Imagery L1, HDF5 (after 1968) data sets. Data are provided in approximately 1-week increments (7 or 8 days) with separate files for the North and South Pole.
Data are provided as Comma Separated Value (CSV) formatted files. Browse images are also available.
NmIcEdg2 data files use the following file naming convention:
Example file name: NmIcEdg2.19188.8.131.520.1.8.south.csv
NmIcEdg2.[YYYY.MM.DD].[yyyy.mm.dd].[north or south].csv
Table 1 describes the file name variables listed above:
|YYYY.MM.DD||Start year, month, and day|
|yyyy.mm.dd||End year, month, and day|
|north or south||Hemisphere|
Data files are approximately 500 KB.
North and South Pole data are provided as separate files that extend from the corresponding pole to 50° N or 50° S. However some regions, for example parts of Alaska, are not available due to technological limitations at the time of the missions.
Roughly 5 km
Intermittent data are available within the following date ranges:
|Nimbus 1||28 August to 22 September, 1964|
|Nimbus 2||15 May to 2 September, 1966|
|Nimbus 3||24 April, 1969 – 16 January, 1970|
Data are provided in approximately 1-week increments (7 or 8 days), however this interval is arbitrary. Users should feel free to merge data files and average over larger intervals.
Data files contain rows of ASCII text, with each row corresponding to an ice edge point. Columns specify latitude, longitude, and edge type plus ancillary information as detailed in Table 3:
|0° – 360° (CW)||50° N – 90° N
50° S – 90° S
|AVCS/IDCS file used to identify edge point.||1 - possible edge (green)
2 - certain edge (red)
3 - edge of lead (magenta)
4 - edge in region of broken ice (orange)
|Horiz. shift (pixels)||Vert. shift (pixels)||Analyst initials (internal use)|
|¹Color denotes edge type in browse images. ²EdgeType = -1 specifies that the reported lat/lon corresponds to the center of an AVCS/IDCS image evaluated for the data file (black squares in browse images). ³Shift applied to better align original image with visible landmarks. See Processing Steps for details.|
Software and Tools
CSV data files can be accessed using spreadsheet software or applications that can read ASCII text.
Data Acquisition and Processing
Ice edges were estimated from AVCS and IDCS imagery obtained by the Nimbus 1, Nimbus 2, and Nimbus 3 satellite missions. See Nimbus Advanced Vidicon Camera System Visible Imagery L1, HDF5: Data Acquisition and Processing and Nimbus Image Dissector Camera System Visible Imagery L1, HDF5: Data Acquisition and Processing for details.
Analysts at NSIDC visually inspected thousands of Nimbus AVCS and IDCS images for the presence of open water and sea ice. Some automated methods were tested; however, the imprecise navigation and inconsistent quality of the original images, combined with the challenge of distinguishing cloud edges from sea ice, required that human observers determine each point by hand. Once identified, points were ranked as either possible or certain based on the analyst's experience and confidence that the edge was distinguishable from clouds (often based on the assumption that ice moves more slowly than clouds). No attempt was made to analyze lake ice or to look for ice over land.
Images were manually shifted by a few pixels from the original navigation to best match obvious visual landmarks such as coastlines. These shifts are stored in the ishift (horizontal) and jshift (vertical) columns of data files.
In regions with broken ice, the edge was estimated using the 15% SSMI threshold, which defines ice extent as the total area covered by all pixels with at least 15 percent ice concentration. This cut-off was adopted to allow comparisons with modern records.
Although every attempt was made to completely categorize all unobscured ice edges in the original AVCS and IDCS images, leads and broken ice regions were not exhaustively evaluated. As such, references to these features in the data are subjective at best and some images that contain broken ice and leads have not been included. Note that the absence of an ice edge may reflect missing data from a given region or time and some weeks and months were not analyzed because the pole of interest was dark.
In addition, although independent contemporaneous sources of Antarctic sea ice extent are lacking, Arctic sea ice charts from the 1960s agree very well with this data set (where the data overlap), lending further credibility to the results.
The greatest source of uncertainty lies in distinguishing homogeneous clouds from homogeneous ice. Fortunately, ice edges are visually different from cloud edges, even in these images from the late 1960s. Furthermore, because clouds tend to move faster than sea ice, viewing multiple images of the same location often differentiates an ice edge from cloud.
The ice edge was assumed to lie between ice and open ocean. This approach works well in the Southern Hemisphere where the ice extends more or less continuously from the pole to the leading edge; however, determining ice extent in the north is more complicated due to islands and regions of open water that lie north of fast moving ice.
As with all derived data, errors in the input can proprogate to the output. The following documents contain details about error sources in original Nimbus images used to produce this data set:
In an attempt to automate ice edge detection, the PIs estimated ice extent by remapping and compositing the minimum brightness for all the images to derive a clear-sky radiance. Although this approach proved inadequate, it does produce a qualitative impression of the ice margin that can be used as independent validation. Visually, the average manually derived ice edge solidly matches the composite minimum brightness image.
Advanced Vidicon Camera System
The Advanced Vidicon Camera System (AVCS) consisted of three earthward-facing cameras deployed in a fan-like array to produce a three-segment, composite picture. Each camera's field of view covered 37 degrees; the center camera pointed straight down while the optical axes of the other two were directed 35 degrees to either side. The cameras utilized an f/4 lens with a focal length of 16.5 mm. A potentiometer attached to the solar array controlled the lens opening from f/16 when the spacecraft was over the equator to f/4 when it was near the poles. Eight-hundred scan-line, 2.54-cm-diameter vidicon pickup tubes yielded a linear resolution of better than 1 km at nadir from an altitude of 800 km. The camera array produced a composite picture covering an area of 830 km by 2700 km.
For additional information about the Nimbus AVCS, see the National Space Science Data Center's Advanced Vidicon Camera System (AVCS) Web page.
Image Dissector Camera System
The Image Dissector Camera System (IDCS) was a shutterless, electronic scan and step tube mounted behind a 108 degree wide-angle, 5.7 mm focal length lens. The camera was installed on the bottom of the satellite sensory ring and pointed vertically down toward the earth at all times. The optical field of view was 73.6 degree in the direction of flight and 98.2 degree in the plane perpendicular to flight. The instrument optics focused the image on the dissector tube's photosensitive surface, while a line-scanning beam scanned the surface at 4 Hz with a frame period of 200 seconds. At the nominal spacecraft altitude of 1100 km, the resulting pictures covered approximately 1400 km on a side with a ground resolution of 3 km at nadir. Scanning and stepping functions occurred continuously as the satellite progressed along its orbital path. Pictures were either transmitted to ground stations in real time or stored on magnetic tape for subsequent transmission.
For additional information about the Nimbus IDCS, see the National Space Science Data Center's Image Dissector Camera System (IDCS) Web page.
References and Related Publications
Contacts and Acknowledgments
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
G. Garrett Campbell
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
The Nimbus Data Rescue Project: Nimbus 1, 2, 3 was supported by NASA contract #NNG08HZ07C as a subtask to NSIDC at the University of Colorado. The PIs also wish to thank Alex Calder, Carl Gallaher, and Anna Schroeder for their contributions to this project, and NSIDC student workers William Harris and Amy Randall.
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