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Map Server User's Guide

Overview

The map server Web tool is designed to permit rapid browsing of the MODIS Mosaic of Antarctica (MOA) data set, and provide readily accessible images for figures, field planning, presentations, etc. This tool was created at the University of New Hampshire by Dr. Mark Fahnestock and Mr. Norman Vine. The MOA Web-based map server contains several pre-stretched versions of the MOA data set, allowing both high-contrast areas (for example, glaciers and mountains) and low-contrast areas (for example, ice shelves, plateau surface) to be displayed with the full information content of MOA.

Note: The map server also allows users to view the RAMP AMM-1 125 m backscatter image from 1997.

Zoom

Use the radio buttons to select zoom options: In, Pan, or Out. Zooming or panning is accomplished by clicking the Refresh/Query button or a location in the large zoomed image. Clicking a location in the small reference image pans to that location at the current zoom level.

The Size field refers to the zoom rate. Zoom is limited to integer steps, 1-25. Note that zooming out by one integer (Use 3 as an example.) and then zooming in by another (for example, 4) gives a fractional increase in zoom (1.33; 4/3=In/Out=1.33).

Layers

Layer Options Description  Range of Surface Brightness Values
Graticule  latitude-longitude grid  
moa_uhc     MOA surface image, ultra-high contrast 15875 - 16125
moa_vhc     MOA surface image, very high contrast 15800 - 16200
moa_hc      MOA surface image, high contrast 15500 - 16500
moa         MOA surface image, nominal contrast 15000 - 17000
moa_lc      MOA surface image, low contrast 12000 - 20000
moa_vlc     MOA surface image, very  low contrast 9000 - 23000
moa_ulc     MOA surface image, ultra-low contrast 1 - 32000
ramp        RAMP  AMM-1 125 m backscatter image 1 - 255

A graticule is a grid of location lines, in this case latitude-longitude lines, that overlays a map or image grid. On the MOA site, this is generated interactively by the Minnesota Map Server (MMS) from the projection information for MOA. The algorithm is set to produce overlay grids with approximately 3 to 10 lines in each direction. Note: There are known problems with the Graticule when it includes the 180° meridian. This layer also lacks line labels at high zoom levels.

The main two-byte MOA data set (namely, the high-pass filtered MODIS Band 1 image) is an array of numbers representing surface brightness, each value of which is in the range 1 to 65535 with an average value of about 16000. However, in order to make a one-byte MOA grey-level image that can be displayed, a portion of this 1 to 65535 range (namely, a minimum and a maximum value) is mapped into values in the range 1 to 255 using linear interpolation. A particular mapping can be represented by a minimum-maximum pair, and is referred to as a "contrast stretch."

The seven MOA image layers are versions of the main MOA data set with different contrast stretches applied. The contrast stretches are applied on-the-fly as the data are extracted from the two-byte MOA image. The stretches allow a user to enhance the features of interest to their application of the data. Low-contrast features, such as subtle ice topography on ice shelves, are best revealed by the moa_uhc (ultra-high contrast) stretch. High-contrast features, such as mountains and valleys, are best represented by the very low or ultra-low contrast stretches of MOA.

The ramp layer is a version of the Radarsat Antarctic Mapping Program (RAMP) Antarctic Mapping Mission 1 (AMM-1) synthetic aperature radar (SAR) image, compiled from data acquired by Canada's Radarsat 1 satellite during September and October of 1997 (for comparison, the MOA MODIS imagery was acquired between 20 November 2003 and 29 February 2004). This 125 m 1-byte product (grey values limited between 1 and 255) reveals details related to both topography and snow characteristics in the upper few tens of meters of the ice sheet (See RAMP Data.).

The Ctrl key can used to select both Graticule, which provides a latitude-longitude grid, and one or two of the image layers at the same time. Selecting two image layers gives a blended image, unless one of them is the ramp mosaic layer, in which case only the ramp mosaic layer is shown.

Upgrades with additional layers and the snow-grain-size image are in the development stage. See the MOA - MODIS Mosaic of Antarctica map server on the University of New Hampshire Web site. (UNH map server availability may be intermittent.)

Comments

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User's Guide

The User's Guide link will direct you to this document.

Interface Tutorial

Follow this example to become familiar with the interface.

  1. To get to the map server, click on the Initialize button on the MOSAIC of Antarctic Data Demo page. There is a large overview of the MOA high-pass image along with a smaller reference image in the upper right.
  2. Click the In radio button under Zoom.
  3. Set the Size to 2.
  4. Highlight moa under the Selected Layer[s] to display.
  5. At the top right-hand side of the page click on Refresh/Query two times.
  6. Assume that this is the view of interest. Click the Pan radio under Zoom.
  7. At the top right-hand side of the page click on Refresh/Query. This step will reset the URL to a standard format.
  8. Right click anywhere in the image and select Save Image As...
  9. Chose a directory, set the file name to moa_test.jpg, and click Save.

Saving an Image

You can use your browser to save a MOA image to your hard drive. For example, using Netscape, move the cursor over the large zoomed image or the small reference image, click the right mouse button, and select Save Image As... You can then specify a directory and file name for a saved JPEG file. A similar procedure can be used with other browsers. Note: Useful information about the image, such as the projected corner pixel x and y coordinates, which is used to derive the latitudinal and longitudinal coordinates, is included in the URL.

URL Information

A great deal of information is available in the URL. It is shown here with carriage returns added to improve readability. The information in bold font is followed by an explanation.

http://nsidc.org/cgi-bin/mapserv?
map_scalebar_status=embed&
zoomdir=0&
zoomsize=2&
layer=moa&
imgxy=400+400&
imgext=-908840.625000+-960065.625000+601565.625000+550340.625000&
map=..%2FMMS%2Fmoa%2Fmoa_wms.map&
savequery=true&
program=%2Fcgi-bin%2Fmapserv&
map_scalebar_status=embed&
map_web_imagepath=%2FWEB%2FDATA%2FTMP%2FMMS%2F&
map_web_imageurl=%2FTMP%2FMMS%2F&
map_web_template=moademo.html

From the URL, the important pieces of information are:

  1. The zoomsize value is 2.

  2. The selected layer is the moa layer.

  3. The imgx and the imgy values are 400 and 400, respectively. Let numcols be the number of columns and numrows be the number of rows, in moa_test.jpg. Then
    numcols = zoomsize x imgx = 2 x 400 = 800
    numrows = zoomsize x imgy = 2 x 400 = 800

  4. The imgext values are llx + lly + urx + ury where

    llx and lly refer to the lower left corner of the lower left pixel:
    llx = -908840.625000 meters
    lly = -960065.625000 meters

    urx and ury refer to the upper right corner of the upper right pixel:
    urx = 601565.625000 meters
    ury = 50340.625000 meters

    It follows that:
    ulx and uly refer to the upper left corner of the upper left pixel:
    ulx = llx = -908840.625000 meters
    uly = ury = 550340.625000 meters

    lrx and lry refer to the lower right corner of the lower right pixel:
    lrx = urx = 601565.625000 meters
    lry = lly = -960065.625000 meters
  5. The above x and y values refer to a projected coordinate system with the center of the projection at the south pole, and include the part of the image "underneath" the scale bar at the bottom of the image.

    If needed, the x and y scales can be calculated using:
    xscale = (urx - llx) / numcols = (601565.625000 - (-908840.625000)) / 800 = 1888.0078 meters per pixel
    yscale = (ury - lly) / numrows = (550340.625000 - (-960065.625000)) / 800 = 1888.0078 meters per pixel

For MOA, investigators employed the Mapx library, which enables the addition of mapping functionality to an application to handle map projections. A map projection is a mapping between geographic latitude-longitude coordinates, usually represented in degrees on the Earth's surface, and Cartesian x-y coordinates, usually represented in meters on a plane surface. Mapx utilities provide access to predefined maps and the capability to define map projection parameters (MPP) stored in a .mpp file. These additional map projection parameters, which are independent of the view, include:

Map Projection: Polar Stereographic Ellipsoid
Map Reference Latitude: -90.0
Map Reference Longitude: 0.0
Map Second Reference Latitude: -71.0 ; latitude of true scale
Map Equatorial Radius: 6378137.0 ; wgs84 meters
Map Eccentricity: 0.081819190843 ; wgs84

This file, Moa.mpp, is available via FTP.

Converting xy corner points to latitude-longitude pairs requires some kind of map projection software package. In the case of the Mapx package (Visit Mapx: Map Transformations Library.), investigators use the xytest program included in Mapx together with Moa.mpp as follows:

xytest Moa.mpp
> assuming map origin lat is same as ref. lat -90.000000
> assuming map origin lon is same as ref. lon 0.000000

forward_mapx:
enter lat lon -

inverse_mapx:
enter x y - -908840.625000 -960065.625000
lat,lon = -77.8762579 -136.5700315 valid
x,y = -908840.6250001 -960065.6250001 valid
enter x y - 601565.625000 50340.625000
lat,lon = -84.4482155 85.2164807 valid
x,y = 601565.6250001 50340.6250000 valid
enter x y - -908840.625000 550340.625000
lat,lon = -80.2439738 -58.8033590 valid
x,y = -908840.6250001 550340.6250001 valid
enter x y - 601565.625000 -960065.625000
lat,lon = -79.6000806 147.9292345 valid
x,y = 601565.6250001 -960065.6250001 valid

So,
lower left lat-lon = -77.8762579 -136.5700315
upper right lat-lon = -84.4482155 85.2164807
upper left lat-lon = -80.2439738 -58.8033590
lower right lat-lon = -79.6000806 147.9292345

Citing These Data

Haran, T., J. Bohlander, T. Scambos, and M. Fahnestock compilers. 2005. MODIS mosaic of Antarctica (MOA) image map. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.

See Also

For assistance or more information on this data set, contact User Services at NSIDC.