Contour Plots

Contour Plots

The ROMS plotting package allows you to plot results for most variables at any depth. You can also overlay a reference field.

Configuration

There are two files that that control the output of the ROMS Plotting Package: default.cnt and the ccnt.in (or csec.in for Cross-section Plots) file. The default.cnt file controls the internal plotting parameters while the .in file determines what data to plot and how it will appear. At first you will not need to edit the default.cnt file so we will start with the .in file.

NOTE: default.cnt and the .in file can be named anything you want. This will become clear as we continue.

We suggest that you copy ccnt.in and default.cnt to the directory where you wish to create your plots.

  > cd ~/MyPlots
  > cp ~/MyDir/src/External/ccnt.in ccnt_hraw.in
  > cp ~/MyDir/Data/default.cnt default.cnt

You may name the file whatever you like. I chose ccnt_hraw.in because I will be plotting temperature. Open the .in file (ccnt_hraw.in in this example) with your favorite editor. The first line of this file allows you to specify the year and starting year-day for your plots. If you do not want a time label make the year-day value less than 0:

  2007 -1      : year and starting year-day (use yearday<0, for no time label)

The next four lines are the plot title. Blank lines are fine but there must be four lines here. The text that you see on lines 2-5 are only a suggestion; you can put any text you like. I will not actually be plotting the title lines but it's good to have for later reference.

  ROMS 3.0
  Philippine Archipelago Straits
  
  Bathymetry (hraw)

The plotting options begin on line six and have short descriptions of what they do. For complete documentation on the plotting options read the Plotting Options page. I will annotate the options section to explain the choices I've made.

I only wish to plot one field and 63 is the field ID for "Raw Bathymetry at RHO-points (m)" (FLDID).

  1     NFIELDS: number of fields to plot. Line below, field(s) types:
  63             field identification: FLDID(1:NFIELDS)

Bathymetry is a special case that has only one level so use the following options.

  1     NLEVELS: number of depths/levels/isopycnals to plot (0 for all levels)
  0              depths (<0), levels (>0) or isopycnals (>1000) to plot

The next three options determine range and frequency of data that will be plotted from your input file(s). The bathymetry (hraw) I want is at the first time record so I have chosen -1 for FRSTD and LASTD. DSKIP is irrelevant since I am only plotting one record.

  -1    FRSTD  : first day (>0) or time record (<0) to plot
  -1    LASTD  : last day (>0) or time record (<0) to plot
  0     DSKIP  : plot every other DSKIP day/record (see below)

FINDX can be ignored for this example but the line MUST remain in the file.

  1     FINDX  : forecast record to process, if any (atmospheric files only).

I am going to stick with linear interpolation so I leave this as "0"

  0     VINTRP : vertical interpolation scheme: 0=linear, 1:cubic splines

I have chosen to set the next two values because I know my range already. If you do not know the range of your data you should enter 0.0 and the plotting package will calculate the minimum and maximum values. Using the defaults can lead to other problems because the minimum and maximum values are recalculated for each plot. This means you could end up with a different range and color bar for every frame, making visual analysis very difficult.

  -6000 PMIN   : field minimum value for color palette (0.0 for default)
  1000  PMAX   : field maximum value for color palette (0.0 for default)

I want contour lines between the color bands so I will set the next option to 1.

  1     ICNT   : draw contours between color bands: 0=no, 1=yes

I am not plotting an iso-surface so I leave this at 0.0

  0.0   ISOVAL : iso-surface value to process (see below)

The next four options control how vectors are drawn. I am not drawing vectors in this plot so I can ignore these.

  1.2   VLWD   : vector line width (1.0 for default)
  4.0   VLSCL  : vector length scale (1.0 for default)
  2     IVINC  : vector grid sampling in the X-direction (1 for default)
  1     JVINC  : vector grid sampling in the Y-direction (1 for default)

The next five values control the overlay or reference field. I want a special overlay of cross-section lines that will be specified in a user created, special file named fort.35. The format of this file will be detailed later in this tutorial. To overlay specified user data from a fort.35 file set IREF to -2. The rest of this section is ignored because IREF is not set to 1 or 2. If you do not wish to overlay anything set IREF to 0. For more information see the Plotting Options page.

  -2    IREF   : secondary or reference field option (see below)
  25    IDOVER : overlay field identification (for IREF=1,2 only)
  1     LEVOVER: level of the overlay field (set to 0 if same as current FLDLEV)
  0.0   RMIN   : overlay field minimum value to consider (0.0 for default)
  0.0   RMAX   : overlay field maximum value to consider (0.0 for default)

This determines the spacing between grid lines in the plot. 2.0 is good for this example.

  2.0   LGRID  : Desired longitude/latitude grid spacing (degrees)

The next four options control the map projection for the plots. I want a Mercator projection so I set IPROJ to 2.

  2     IPROJ  : map projection (see below).
  0.0   PLON   : projection Pole longitude (west values are negative).
  0.0   PLAT   : projection Pole latitude (south values are negative).
  0.0   ROTA   : projection rotation angle (clockwise; degrees).

I want color contours on land so I do not want to color mask the land. The levels are discussed in the Plotting Options page.

  0     LMSK   : flag to color mask land: [0] no, [>0] yes  (see below)

I want one plot per page and I want to preserve the plot aspect ratio so I set NPAGE to -1.

  -1   NPAGE  : number of plots per page (currently 1, 2, or 4)

I don't want grid positions computed from the specified grid or history file.

  F     READGRD: logical switch to read in positions from grid NetCDF file.

I don't know what this does.

  F     PLTLOGO: logical switch draw Logo.

The next 5 options control plot titles captions and legends.

  F     WRTHDR : logical switch to write out the plot header titles.
  F     WRTBLAB: logical switch to write out the plot bottom title.
  F     WRTRANG: logical switch to write out data range values and CI.
  F     WRTFNAM: logical switch to write out input primary filename.
  F     WRTDATE: logical switch to write out current date.

They are several coastline files in the Data directory of the source code but I have a special one for my plot that will be specified later.

  T     CST    : logical switch to read and plot coastlines and islands.

If you leave these values at 0.0 the plotting package will take the entire domain of your output file. I will set the domain I want to plot.

     6.0  14.0  : bottom and top map latitudes (south values are negative).
   118.0 128.0  : left and right map longitudes (west values are negative).

The next seven lines specify the input files for the plots. The first line is the variables ID file. An example file exists in the Data directory of the plotting package source code and I will use it.

  ~/Mydir/Data/varid.dat

The second file sets the color palette file. Several palettes are included in the source code Palettes directory. I'm going to use the gebco1.pal file.

  ~/Mydir/Palettes/gebco1.pal

The third file sets the internal plotting parameters. We will discuss this file in detail later. An example file exists in the Data directory and I will use it.

  ~/Mydir/Data/default.cnt

The fourth file is your primary NetDCF file. I have my bathymetry in a separate NetCDF file.

  philip_hraw_b.nc

The fifth file is you secondary NetCDF file. Some times the data you want to overlay will be in a different file. If that is the case you need to set IREF to 2, then put the name of the file your overlay data should come from here. However, my overlay data is in a special fort.35 file so it doesn't matter what I set this to so I'll set it to be the same as my primary NetCDF file. For more information about IREF check the Plotting Options page.

  philip_hraw_b.nc

The sixth file is your grid NetCDF file. As mentioned above, I don't want my grid positions computed from a grid file so it doesn't matter what I put here because it will be ignored. I will use my primary NetCDF file again.

  philip_hraw_b.nc

The seventh file is you coastlines file. There are several coastline files in the Data directory. My coastline file is named philip_high.cst. If you don't want or need coastlines (for example when plotting the upwelling test case), set CST to F and this line will be ignored.

  philip_high.cst

Save the ccnt_hraw.in file. Now we just need to create our fort.35 file specifying our cross-section lines.

fort.35

Create a new file named fort.35 and open it with your favorite editor. The format of a fort.35 is pretty simple. Each shape block begins with 999.000000 then an RGB color specification (i.e. 0 255 0 would be green).

   999.000000                  0  255    0

all lines between the above line and the next shape block beginning line (999.000000) are ordered pairs of points. You need at least 2 points for anything to show up. The following 3 lines tell the plotting package to overlay a horizontal green (0 255 0) line from 8.8°N, 118°E to 8.8°N, 128°E.

   999.000000                  0  255    0
     8.800000   118.000000
     8.800000   128.000000

I have 6 cross-section lines to draw so my fort.35 file looks like this:

   999.000000                  0  255    0
     8.800000   118.000000
     8.800000   128.000000
   999.000000                237  107  172
    11.300000   119.650000
    11.300000   122.010000
   999.000000                153    0  204
    12.200000   120.250000
    12.200000   121.050000
   999.000000                255  255    0
     6.000000   121.300000
    12.250000   121.300000
   999.000000                255  255  255
     8.550000   123.100000
     9.030000   123.100000
   999.000000                255    0    0
     9.000000   125.320000
    10.500000   125.320000
   999.000000   999.000000

The 999.000000 999.000000 line tells the plotting package there are no more shapes to draw. You can draw any geometric shape by specifying more points in a shape block. For example, if you wanted to draw a red box you would make a block like the following:

   999.000000                255    0    0
     9.000000   125.320000
    10.500000   125.320000
    10.500000   124.000000
     9.000000   124.000000
     9.000000   125.320000
   999.000000   999.000000

Plotting

Now that we have created an input file for ccnt we can start creating our plots. First, let's make sure ccnt and idt are in our path and the NCARG_ROOT environment variable is set.

  > which ccnt
  /home/robertson/bin/ccnt
  
  > which idt
  /home/robertson/ncl-5.0.0/bin/idt
  
  > printenv NCARG_ROOT
  /home/robertson/ncl-5.0.0

if your shell cannot find ccnt you need to add PLT_BINDIR to your path. the NCARG_ROOT environment variable should be set to the location of your NCL install. See the Installation page for more details. If you have an existing 'gmeta' file that you want to keep you should first rename it so ccnt does not replace it. To create the plots execute the following command.

  > ccnt < ccnt_temp.in

You will see some information about each frame scroll by. When the program finishes you will have an NCAR Computer Graphics Metafile (NCGM) named gmeta in the current directory. Now you can use idt to view your plots.

  > idt gmeta &

This will bring up the idt viewer with a blank plot. To see the first frame click the button. My bathymetry plot is shown below.

Clicking the button repeatedly will advance to the frames of the gmeta file if you plotted more than one record and/or field. I suggest you take some time to experiment with the controls in the idt window.

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