June 1, 2004 News

The Ad Hoc Format Working Group has completed its formal business, with efforts leading to a new WMO JCOMM-sanctioned format. The descriptive document may be referenced as:

SIGRID-3: A Vector Archive Format for Sea Ice Charts. JCOMM Technical Report Series No. 23, 2004, WMO/TD-No. 1214.

Ad Hoc Format Working Group -- Shapefile Discussion

Information on Shapefiles
Shapefiles from DMI -- A Format to Consider

Information on Shapefiles

ESRI Shapefile Technical Description

An ESRI "white paper" defines the shapefile data format. It also provides all the technical information necessary for writing a computer program to create shapefiles without the use of ArcView GIS or other ESRI software for organizations that want to write their own data translators. (126 KB/34 pages. From the ESRI web site)

Shapefiles versus Coverages:

This document compares ESRI vector format options (4/01 - courtesy Awtar Koonar, A/Chief - Information Technology Division, Canadian Ice and Marine Service, Meteorological Service of Canada)

Viewing Shapefiles:

There is a freeware package called ArcExplorer which may be downloaded from ESRI - you may use it for display of shapefiles. ERDAS has also its own freeware viewer which can display shapefiles. (9/00 - H. Andersen)

Definition of Shapefile Elements:

(9/00 - from ArcView online help) :
ArcView shapefiles are a simple, non-topological format for storing the geometric location and attribute information of geographic features. A shapefile is one of the spatial data formats that you can work with in ArcView.

The shapefile format defines the geometry and attributes of geographically-referenced features in as many as five files with specific file extensions that should be stored in the same project workspace. They are:

  • .shp - the file that stores the feature geometry.
  • .shx - the file that stores the index of the feature geometry.
  • .dbf - the dBASE file that stores the attribute information of features. When a shapefile is added as a theme to a view, this file is displayed as a feature table.
  • .sbn and .sbx - the files that store the spatial index of the features. These two files may not exist until you perform theme on theme selection, spatial join, or create an index on a theme's Shape field. If you have write access to the source data directory, the index files will be persistent and remain after your ArcView session is complete. If you do not have write access to the source data directory, they will be removed when you close the project or exit ArcView.

Shapefiles from DMI -- A Format to Consider

At the October 2000 IICWG meeting, the ad hoc format working group proposed the following. (Since then, the U.S. Canada Joint Ice Working Group has also endorsed shapefiles as an archive format, and the WMO JCOMM Expert Team on Sea Ice is considering the shapefile format.)

  1. Those ice centers using GIS software for ice chart production will submit files to the GDSIDB data bank in ESRI Shapefile format.
  2. Ice attributes attached to individual polygons will be encoded using SIGRID. Additional fields and codes may be defined as appropriate. (Attributes can differ from center to center)
  3. A metadata file will be used to describe the content of the shapefile: source of information, dates covered, and other descriptive information. (Metadata should be standard across centers)
  4. The format for the attributes and metadata will be decided by a format working group

In March 2001, NSIDC received all of the Danish Meteorological Institute's year 2000 Greenland ice analysis charts in shapefile format. Chuck Fowler undertook an effort to see how easy the DMI shapefiles were to work with, from a scientific data center's perspective of wanting to archive and provide sea ice chart data in a form that researchers could easily use. 

In late April, NIC and CIS also provided sample shape files to NSIDC for a quick comparison. Although Chuck could work with all files there were minor differences between them. The files from NIC and CIS were provided without any effort in advance to standardize format or content, and were provided at NSIDC's request simply to give NSIDC more experience dealing with Shapefiles in advance of the May JIWG meeting.

A Shapefile Format for IICWG And GDSIDB?

Please evaluate the DMI shapefiles with a view toward providing shapefiles from your center to the WMO Global Digital Sea Ice Data Bank in the DMI or a similar shapefile format. Send your comments to Florence Fetterer (fetterer@nsidc.org). I will compile comments, and we will discuss them at the November 2001 IICWG meeting.

Please comment on the metadata file format. We would like to have the format of this file be consistent across ice centers.

It is not necessary to have a consistent format for the other files. However, there are some DMI file characteristics that are helpful in a programming/archiving context:

  • DMI files have shape specifications are in lat/lon coordinates. This makes it a simple matter to convert the files to a gridded format (NSIDC usually uses the EASE-Grid format.) Since each center may use different map projections, spheroids, false eastings/northings, etc., it is a great help if the locations are in lat/lons rather than image coordinates (as the CIS and NIC samples were). However if it is not possible to produce locations with lat/lon positions, positions can be converted to lat/lon if the projection and other ancillary information is known.
  • DMI files have shape attributes are in separate columns. From a programmers standpoint, it saves having to parse the SIGRID strings such as, for example, "CT00CA309399CB209199". (4/01 - C. Fowler)

DMI Shapefile Documentation:

DMI shapefile documentation consists of tables describing object attributes.

DMI Shapefile Examples:

DMI ice chart shapefiles from 6 March 2000. These correspond to the files C. Fowler converted to EASE-Grid format.

The DMI 'sym' files contains points (symbols) e.g. position and attributes (type and angle) for open water symbols, ice bergs symbol etc. They are included to enable a complete reconstruction of the chart but are of no use if polygons are to be analysed. (4/01 - H. Andersen.)

The DMI metadata file (.md):

A DMI chart metadata file example (file 2000031212_00_AREA.MD) is given below. The documentation (Table 8) defines the values.

Datum:WGS 84;
Spheriod:WGS 84;

Please provide comments on this metadata file format.