The following is an elevation data set for the Canadian Maritime region downloaded from the US Department of Commerce/NOAA/National Geophysical Data Center World Data Centers A for Marine Geology and Geophysics and Solid Earth Geophysics at:
                 NGDC/WDC-AMGG_SEG-5Minute Gridded Elevation Data Selection Help
                 (http://www.ngdc.noaa.gov/mgg/global/seltopo_help.HTML#geosel)
     The data set and header notes are contained in the file mkimage.txt.
     Make a hard copy note of the minimum and maximum latitudes (44,51), longitudes (-64,-49), number of columns (181) and rows (85), the fact that the data file is an Ascii file, and that the values are integer values.
     What is the relationship between the longitude values -64, -49 and the column value 181, and why 181 and not 180? [ 180 = (64-49)*60/5] There are 180 5-minute cells between longitudes -64 and –49 (as there would be, for example, two one degree cells between longitude values of 1 and 3.) However, the coordinates of a cell are given by the coordinate position of the bottom left corner of the cell, and therefore a cell must exist for each coordinate position along the topmost latitude and the right hand longitude boundaries of the area. A cell must exist for the top rightmost coordinate position 51, -49, and the number of cells must therefore be equal to the number of cells between 64 degrees and 49 degrees (180) plus 1 ( 180 + 1 = 181); that is, an extra row and column of cells needs to be specified to correspond to the right- and top-most coordinate positions. The bottom left location of the image and the cell at this location will have the coordinates -64, 44, but the true latitudinal boundary of the image will be at 51 + .083333 degrees (51 + 1 cell) and the true longitudinal boundary will be at -49 + .083333 degrees (-49 + 1 cell).
    The resolution of the image, the cell dimension, will be .08333 degrees (5/60). Note one 5 minute cell would have a resolution of about 6 kilometers (40030/360/12*cos 48).

    PROCEDURE

    1) Go to your folder in ‘users on ‘Earthnt’ (H:)’, where ‘H’ is the name of the mapped drive Earthnt\Users on your computer (it could be some letter other than ‘H’, or it may not exist and will have to be created), and make a folder named ‘your initialsmarit’ KEEP READING! - (do not include the quotes, and ‘your initials’ really means your initials, e.g. wrc1, and not the string ‘yourinitials’). If you have not previously used the computers in room 17 and are baffled by this instruction, do not panic and politely request the help of the instructor.
     Create a window corresponding to ‘Earthnt\Public\Es300b and copy the folder Earthnt\Public\ES300b\marit’ into the folder you have just created by clicking and dragging the ‘marit’ folder from the ‘Public’ window into the ‘Users’ window.

     Open the file   mkimage.txt   in Excel or your Word processor (Windows Word, Wordperfect, Write) and remove the header notes AND THE LATITUDE AND LONGITUDE COLUMNS, and save the file as a text file maritasc.txt. If you remove the lat longs, how will the software know where to plot the data?
     Copy the file   maritasc.txt   to   maritxyd.img   to be used as an IDRISI image file. It has happened that the the renamed file has ended up named maritxyd.img.txt!! If this should happen, the file may be safely renamed to maritxyd.img from within 'File Maintenance' (in FILE) in IDRISI.
     Load Idrisi and use DOCUMENT in FILES to create a   maritxyd.doc   document file describing   maritxyd.img   , entering 181 for "Columns", 85 for "Rows", integer for "Data type", ascii for"File type", latlong for "Reference system", degrees for "Reference Units", -64 for Min X, -48.916667 for Max X, 44 for Min Y, 51.083333 for Max Y, and 1 (degree) for the unit distance. Click the "Calculate" buttons to insert the Maximum and Minimum data values, and the Resolution (size of the cell in degrees = 5 minutes = 5 * .0166667 degrees = .0833333 degrees).
     Display the image file with Display/DISPLAY Launcher (or press F5), accepting, when requested, the offer to convert the Ascii file to a binary file.

     To display a draped Ortho image:
     Run STRETCH to generate a stretched image   maritst.img   from   maritxyd.img (m1.img). Ask the instructor about the meaning of STRETCH.
     Run ORTHO in DISPLAY with   maritxyd.img   as the surface image and   maritst.img   as the drape image. Select the output resolution as 1024 x 768.

     Check the histogram for the maritxyd.img, and the depths in the St Lawrence channel. Make a stretched image of depths between -200 and -500 metres.

    Metadata for the image:
    Area:
       51 North latitude
        |
     -64 West longitude  |  -49 West longitude
        |
      44 North latitude

    Data format:  Datavalues in whole Meters (- (negative) = below sea level)
    Delimiter: ","
    Grid size: 181 x 85 matrix of 5-minute grid values beginning
           at NorthWest corner progressing Eastward for 181
           values, then stepping 5 minutes South for the next
           row, ending at SouthEast corner.
    There are 15385 (=181x85) rows of data values
    The cell resolution is 5 minutes or 1/60*5 = .0833333 degrees
    Note: Negative latitudes are South; Negative longitudes are West.
    e.g.
    Lat,    Lon,    Datavalue
    51.00,-64.00,610
    51.00,-63.92,541
    51.00,-63.83,442