ncview man page

ncview — graphically display netCDF files under X windows


ncview [-beep] [-copying] [-frames] [-warranty] [-private] [-ncolors XX] [-extrainfo] [-mtitle "title"] [-minmax fast | med | slow | all] datafiles ...


Ncview displays 2-D slices of a netCDF data file, using the X Window System graphical user interface (Release 4 or higher). You can examine different floating point variables in the file, and animate the floating point data along the “record dimension” (usually time) to see how it evolves. You can also display 1-D (line plot) views of the data simply by clicking the mouse on the point of interest.

When you first invoke ncview, a command panel comes up which has a number of buttons for manipulating the current view into the data file, and presenting various information about the current view. From the top, going down, the information fields are: the 'title' of the data file; the 'long_name' of the currently selected variable; the frame number (i.e., place along the scan axis) currently displayed; the minimum and maximum values of the variable; and the value of the data point under the cursor (only active when the pointer is over the color contour image).

Next comes a row of buttons similar to a tape recorder, used for changing the view into the netCDF file along the scan dimension. In Version 1.XX of ncview, the scan dimension is constrained to be the “record dimension” (in netCDF parlance). From the left, the buttons are: the quit button; a button to take you directly to the first frame, marked "->1"; rewind, which loops the images going backwards; step backwards; pause; step forwards; and fast forward, which loops the images going forwards.

Below this is the row of option buttons, which from the left are: the colormap button, labeled with the name of the current colormap (see below); "Inv P", which inverts the physical representation of the data (flips it upside-down); "Inv C", which inverts the colors currently being used so that the colors indicating minimum and maximum are switched; the magnification button, which sets how much image expansion the image undergoes; and the transformation button, which determines what preprocessing the data undergoes before display. For this button, "Linear" means no preprocessing, "Low" means that the data is raised to the fourth power before conversion to a pixel, so that low values are emphasized; and "Hi" means that the fourth root of the data is taken before conversion, so that large values are emphasized. Next comes "Set Dim"; pressing this pops up a window which allows you to determine which variables are shown on the X and Y axes. Note that Version 1.XX of ncview will not transpose your data! This means that, for example, you cannot simultaneously display the X dimension along the Y axis while displaying the Y dimension along the X axis---that would be an attempt to transpose the data. You can display the X dimension along the Y axis if some other variable which varies less rapidly in your particular data file (for example, depth) is on the X axis. Such a configuration is possible because it involves no transposition of data. In general you don't have to worry about this issue much, because if you attempt to pick axes which would be transposing the data, ncview switches them (and tells you that it's doing so!) so you can get the axes you want. Note that there is never any ambiguity about which dimensions are being displayed on what axes; that information is always shown in the main panel. Next is "range", which pops up dialog boxes to set the data min and maxes which will be contoured. Pressing with the RIGHTMOST mouse button on the "range" button resets the ranges to match the currently displayed slice; this is a VERY useful option, so remember it and make use of it frequently! The last button shows the method currently employed for expanding the data onto the screen; the default, "bi-lin", performes a bi-linear interpolation. Also available is "repl", which simply replicates the pixels and is somewhat faster.

The next row of buttons shows what variables can be displayed from the input files. Note that when ncview first comes up, if there is more than one variable in the file, you must select a variable to display before you will see anything. If there is only one variable in the file, the selection defaults to that one.

Below the variable selection buttons are the dimension information fields. All the dimensions for the displayed variable which can take on more than one value are shown here, one variable to a line. In each line, there are 6 fields of information; from left to right, they are: "Dim", the Dimension identifier, which is `Scan' if the dimension is currently the scanned dimension (i.e., the dimension accessed via the tape-recorder style buttons), `X' if the dimension appears in the color contour display along the x axis, or `Y' if it appears in the color display along the y axis. This field will be blank if it isn't Scan, X, or Y. Next come "Name", the dimension's short name; "Min", the minimum value of the dimension; "Current", the current value of the dimension as displayed in the color contour panel; "Max", the maximum value of the dimension; and "Units", the dimension's units. Clicking on the "Current" field of a dimension allows you to change the current value of that dimension. Clicking with the left mouse button increases the current value of that dimension; clicking with the right button decreases it.

Udunits Support

ncview supports time axes that use the conventions in the udunits package. Typical units names in this scheme would be "days since 1990-01-01". If ncview encounters a time dimension that it understands in this way, then it displays the calendar date (as calculated by the udunits package, not ncview) rathar than the actual axis value. For instance, it might display "3_Jun_1995" rather than "Day 2390". To have this functionality, the udunits package must be able to find the "udunits.dat" file. You must set the environmental variable UDUNITS_PATH to the location of this file for ncview to be able to find it.


Clicking on a button with the left mouse button invokes the standard action described above; clicking with the right mouse button on the colormap select, transformation, magnification, or dimension "Current" buttons DECREASES the selection instead of increasing it (i.e., cycles in the reverse direction). Holding down the control key "accelerates" actions; while clicking with the left mouse button will increase the rate at which the rewind, step backwards, step forwards, and fast forward keys will step through the data. When holding down the control key while clicking on the magnification button, the magnification DOUBLES or HALVES instead of incrementing or decrementing by one.

Ncview attempts to save the displayed images in main memory, with each frame being saved as it is calculated for the first time. This speeds up looping replays of the same data. If there is not enough memory to store all the required frames at the selected magnification, ncview will inform you and automatically stop trying to do so. Changing the magnification will again force ncview to try and allocate a image buffer.

Since the scaled, interpolated pixel maps are stored, the following operations will flush the image buffer and require recalculating the images if they are performed: inverting the data; inverting the color map; changing the magnification; changing the data transformation (linear, lo, or hi); changing the dimension; changing the range; changing the pixel replication scheme. Changing colormaps does not require refilling the image buffer.

You can invoke ncview with multiple netCDF filenames on the command line, and it will try to present the data in a logical way; i.e., if there are identically named variables in the data files, it will try to treat them as if they were all in one giant data file. If there are different variables in different files, it will let you choose to display any of the available variables. This is generally a Good Thing, but if you have identically named variables in different files with different attributes, ncview will not know which attribute you want to use and most likely will crash.

Setting the Data Range

It is important to set the data range correctly; otherwise, the color contour might come out all red, or all blue, or otherwise not very interesting. There are a number of ways to set or manipulate the range: 1) Click with the left mouse button on the "range" button. This pops up a dialog window letting you specify the minimum and maximum values directly. 2) Click with the right mouse button on the "range" button. This scales the displayed data to the currently shown frame. 3) Click with the left mouse button on a data point in the color-contour window; this will set the minimum scaling to the value of the data which you clicked on. 4) Click with the right mouse button on a data point in the color-contour window; this will set the maximum scaling to the value of the data which you clicked on.


-beep: rings the terminal's bell when stepping forward through frames in movie mode and the loop is restarted.

-extrainfo: Puts up extra information in the color-contour window. This is useful for photographing the computer screen to make slides or pictures of the data.

-frames: This will make ncview dump out the frames it displays in a series of PPM-format files. You can then make them into an mpeg movie if you so desire (using tools other than ncview).

-mtitle: Puts the following argument (enclosed in quotes) up as the title of the color-contour window.

-ncolors: Sets the number of colors which will be displayed. Defaults to 200. Must currently be less than 256.

-private: Forces use of a private colormap. This will cut down on the number of colormap entries used, but will turn the rest of the screen annoying colors.

-minmax: determines how the calculation of minimum and maximum values is done. If fast, then only the first, middle, and last time entries of each variable are examined. If med, then every fifth time entry is scanned for extrema. If slow, then every tenth entry is used. If all, then every time entry is examined for extrema. Default is "fast".

-copying: prints out the terms under which ncview may be copied, distributed, and modified. Ncview is covered under the provisions of the Gnu General Public Liicense Version 1.

-warranty: Ncview comes with no warranty; this option prints out a fuller statement to this effect.


Ncview looks in directory /usr/share/ncview for system-wide colormap (.ncmap) files. It also examines the user's environmental variable NCVIEWBASE for the name of a directory which contains additional colormap files. If that is not defined, then colormaps are sought in the user's home directory, and in the directory which was run from.

Colormap files have 256 lines, each consisting of one r g b triplet, where r, g, and b are integers in the range of 0 to 255. There should be only whitespace separating the r, g, and b values on each line. Colormap files end with the extension ".ncmap". If Ncview does not find any colormaps, it will complain, and supply a simple default map.

It is necessary to install the applications default file, "Ncview", in your $XAPPLRESDIR directory for the program to function properly. If the screen appears out of alignment, make sure that this installation has been performed.

The application resources file recognizes the following resources, in addition to the standard ones:

The width, in pixels, of the information labels at the top of the main window. If you generally use long titles and variable longnames, you might want to increase this. Default = 400.
The width, in pixels, of the "variable" and "dimension" buttons. If you use long names for these, you might want to increase this value. Default = 50.
The number of variable buttons in a row before a new one is started. Set to be aesthetically pleasing to you. Default = 5.
The amount to step forward and backwards by when the control key is held down while pushing the button. If this value is less than 0, in indicates an absolute number of steps to take; if this value is greater than zero, it indicates the percent (in integer form, from 1 to 100) of the total file size to step. Default = 10 (ten percent).


Occasional bugs surface, especially when mixing variables in different files.

Please send all bug reports to pierce@cirrus.ucsd.edu