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Polygons, Polymarkers, Polylines, Text

This page describes how to use various primitive drawing routines to add markers, lines, filled areas (polygons), and text on an existing plot or anywhere outside a plot (known as "NDC space" or the "frame").

Drawing primivites on a plot

To draw primitives on top of a plot, you must use the plot's data space. On a map plot, for example, you would use lat/lon values to position the primitives. The routines for drawing primitives on a plot are:

gsn_add_polyline   gsn_polyline
gsn_add_polymarker   gsn_polymarker
gsn_add_polygon   gsn_polygon
gsn_add_text   gsn_text

The difference between the "add" versions of these routines (which are functions) and the other routines (which are procedures) is that the "add" functions actually attach the primitive to the existing plot, and you won't see the primitive until you actually draw the plot. If the plot is resized, the primitive will be resized accordingly. This can be useful if you plan to use gsn_panel to panel the plot later and want the primitives to be resized.

Important note: since these are functions, you need to make sure that every return value is unique. If you reuse the same variable to add primitives to a plot, then only the last primitive that you add will be visible. See example "polyg_4.ncl" below.

The procedural versions of these routines simply draw the primitive when you call procedure, and do not attach the primitive to the plot. These routines are a little easier to use because you don't have to worry about the return value. They can also be faster if you are drawing tons of primitives, because less memory is used.

Drawing primivites on the NDC square

To draw primitives on the NDC square or frame, you must use values from 0.0 to 1.0 for the location of the primitive(s). Location (0,0) represents the lower left corner of the square, and (1,1) represents the upper right corner of the square. The procedures for drawing in NDC space are:

gsn_polyline_ndc
gsn_polymarker_ndc
gsn_polygon_ndc
gsn_text_ndc

Note that there are no "add" versions of these routines, because there is nothing for which to attach the primitives.

Useful tip: the drawNDCGrid procedure draws a grid and labels the NDC locations. This is very useful to help you position primitives in the NDC square. See example "polyg_18.ncl".

Potential backwards-incompatible change

There is a potential incompatible change with gsn_polyline, gsn_add_polyline, gsn_polygon, and gsn_add_polygon in NCL version 6.2.0 and later, when attaching lines or polygons to a map.

Previously, drawing a polyline around the equator, for example, could be specified using 2-element arrays. For example:

    lnid = gsn_add_polyline(wks,map,(/0,360/),(/0,0/),lnres)

Now, however, in order to eliminate a number of ambiguous situations and to make user code simpler in most cases, a new behavior has been introduced: the line between two points on the globe always follows the shortest path. In the example above, the behavior in NCL V6.2.0 leads to a 0-length line. The recommended approach now for drawing a line around the equator is to use four points, such that the distance from one to the next is always less than 180 degrees. For example:

    lnid = gsn_add_polyline(wks,map,(/0,120,240,360/),(/0,0,0,0/),lnres)
polyg_1.ncl: This code comes from Mark Stevens of CGD NCAR. To teach himself NCL, he decided to try and plot the climate divisions that are used by the NCDC. Outstanding job Mark! By the way this is the most exciting plot to watch as it draws. Each state is done separately. Can you name each state as it draws?

gsn_polygon_ndc will add polygons in ndc(page) coordinates while gsn_polygon will add them in plot coordinates.

The "climdiv_polygons.nc" netCDF file used in this script is available here (767440 bytes).
polyg_2.ncl: This is the same code as example one, except that Mark is now coloring each polygon with a precipitation value.

mpGridAndLimbOn = False, turns off the lat/lon grid.

mpAreaMaskingOn = True, enables area masking. This then allows the map to be divided into different areas by setting the resource mpFillAreaSpecifiers, e.g. (/"Water","Land"/). These designated areas can then be filled by setting the resource mpSpecifiedFillColors to various colors. In this case, they were set to zero to mask them entirely.
polyg_3.ncl: A plot with poly markers added. You would be able to panel this plot. There are numerous marker styles to choose from.

gsn_add_polymarker is the plot interface that will add polymarkers to a plot so that they can be paneled. There is also gsn_polymarker and gsn_polymarker_ndc which adds the polymarkers in page coordinates.

IMPORTANT: note the syntax on the use of this function:
dum1 = gsn_add_polymarker(wks,plot,glon(inds),glat(inds),polyres) . With this function, you need to set it equal to some sort of dummy variable. Do not set it equal to plot like we do in all other cases. Also, if you do panel this type of plot, do not delete that dummy variable or over write it.
polyg_4.ncl: Draw and fill a box using polylines and polygons

gsn_add_polyline is the plot interface that adds polylines to a plot, and gsn_add_polygon is the plot interface that adds polygons to a plot. This version allows for panelling. There is also gsn_polyline and gsn_polyline_ndc which adds polylines in page coordinates.

gsLineColor = "red", changes the line color to red.
gsLineThicknessF = 2.0, changes the thickness of the line.
gsFillIndex > 0 changes the fill pattern to a pattern (rather than a solid).
gsFillScale changes the density of the fill pattern.
gsLineLabelString = "test", is one method of labeling a box of this type.
polyg_5.ncl: Demonstrates the use of a polygon to shade an xy curve.
polyg_6.ncl: Demonstrates adding polylines.

The trick with this plot is to create an array of dummy graphic variables. When you use gsn_add_polyline, the result must be a graphic variable. In a loop, you must not over write the dummy variable, which is why we need an array.
polyg_7.ncl: Demonstrates how to create your own polymarker.

As of NCL version 4.2.0.a030, you can make your own marker using NhlNewMarker. You give the function the character and font table you want the marker taken from, and provide sizing and placement values. The function returns a marker index that can be used with xyMarkerColor.
polyg_8.ncl: This example shows how to plot values at station locations using different colors and marker sizes for each station point. The station values are grouped depending on which range of values they fall in, and then every marker in this group gets the same color and size.

A legend is added at the bottom, using calls to gsn_polymarker and gsn_text_ndc.

polyg_8_lbar.ncl: This example is very similar to the previous one, except a labelbar is drawn instead of a legend.

The gsn_create_labelbar and gsn_add_annotation functions are used to create the labelbar and then attach it as an annotation. This allows you to use it in a call to gsn_panel if desired.

polyg_9.ncl / polyg_shp_9.ncl This example shows how to draw the Meteorological Subdivisions of India. The first script draws the subdivisions given the boundary coordinates for each subdivision in an ASCII file. The second script draws the subdivisions from a shapefile.

In the first script, the left plot uses gsn_add_polyline to attach the subdivision outlines to the base map. The right panel uses gsn_add_polygon add filled subdivisions.

The first script produces two images: the first image uses NCL's map outlines to draw the states of India (USE_SHAPEFILE_OUTLINES=False). The second image uses outlines from a shapefile (USE_SHAPEFILE_OUTLINES=True).

In the second script, the left panel uses gsn_add_shapefile_polylines to add the subdivision outlines and the right panel uses gsn_add_shapefile_polygons to add filled subdivisions. It draws India outlines from the same shapefile as the second frame of the first script.

The third image with the India outlines and subdivisions coming from shapefiles seems to be the best "match" between the two sets of outlines.

polyg_10.ncl: This example shows how to draw various polylines and polygons on a several generic tickmark backgrounds to create a series of bar charts. The gsn_add_polyline and gsn_add_polygon functions are used to create the polylines and polygons and gsn_panel is used to panel all the plots on one frame.
polyg_11.ncl: This example shows how to draw the political divisions of Brazil. It was contributed to us by Mateus da Silva Teixeira from IPMet, who gave us permission to include it and the data files. It is based on example 9 that draws the Indian subdivisions.

The data files can be downloaded via the estados_brasil.tar tar file. You need to type "tar -xvf estados_brasil.tar" to extract the files.

In version 5.1.0, you will be able to generate the states of Brazil using a new NCL map database. See example 16 on the "maps only" page.

polyg_12.ncl: An example of "layering" different plot objects to get a particular effect.

As the extratropical polygons (red/blue) require the ocean to overlay the polygons, and the tropical polygons (green/yellow) require the land to overlay the polygons, the only way to get the desired effect is to draw two plots, and to set tfPolyDrawOrder equal to Draw.

A third blank plot is drawn over the previous two to draw the border correctly.

This is a reproduction of figure 1b from Kutzbach et al. 2008, Climate Dynamics, 30:567-579.

polyg_13.ncl: An example showing how to overlay filled boxes on a map using different methods:

The third method is the preferred one if you want straight edges for your polygon and you want to be able to attach the box to your map (for paneling or resizing later).
polyg_14.ncl: Draws a line on an existing map between two points on the globe using two different methods: (a) Setting mpGreatCircleLinesOn=True, (b) using gc_latlon to create a great circle path between two locations. Uses gsn_add_polyline to add the polyline. Any map projection can be used.

polyg_15.ncl: This is a minimal script to plot the location of stations on a map background.
polyg_16.ncl: This is a simple script to color plot the location of stations on a map background and manually add a legend. (Donated by Lisi Pei.)
newcolor_4.ncl: This example shows how to draw partially transparent markers, using the gsMarkerOpacityF resource introduced in NCL 6.1.0.

The first frame is drawn with fully opaque markers (gsMarkerOpacityF=1.0), and the second frame is drawn with slightly transparent markers (gsMarkerOpacityF=0.7).

newcolor_12.ncl: This example shows how to draw partially transparent filled polygons using the gsFillOpacityF resource introduced in NCL 6.1.0.

The point of this example is to show how various boxes look when they overlaid in a different order. The middle column was drawn starting with the red box starting first. The right column was drawn with the yellow box starting first.

polyg_17.ncl: This script was written mainly to show how to read an ASCII file with groupings of headers and data.

It then creates a map of color-coded surface wind speed values based on an array of levels. A new function called span_color_indexes is used to select a nice span of colors through the given color map. This function was added in NCL version 6.2.0.

scatter_10.ncl: Demonstrates how to overlay a scatter plot (of filled squares) on a map plot, when the scatter plot is not in lat/lon space. The key is to use gsn_csm_blank_plot to create a canvas for drawing the filled polygons, making sure that the four corners of the blank plot correspond with the four corners of the cylindrical equidistant map plot that is created.

You then set tfDoNDCOverlay to True to make sure that when the blank plot is overlaid on the map plot using the overlay procedure, it simply lines up the corners of the two plots and does the draw.

In this example, the blank plot goes from 0,ny+1 in the Y direction, and 0,nx+1 in the x direction. This corresponds to the following lat/lon locations:

     x = 0    --> lon = -180
     x = nx+1 --> lon =  180
     y = 0    --> lon =  -90
     y = ny+1 --> lon =   90
   
polyg_18.ncl: This script is an overall demonstration of the various primitive drawing routines, both for adding or drawing primitives on a plot, and for drawing primitives in NDC space.

The first frame shows primitives drawn using the map plot's data space (lat/lon values).

Note the use of unique_string to generate a unique attribute name, which is then attached to the existing map variable as a way of creating a unique name for each marker primitive.

An NDC grid is drawn using drawNDCGrid. This was used to determine the NDC locations for the primitives in the second frame of this script.

polyg_19.ncl: This script shows how to draw a map of mainland United States with Alaska, Hawaii, and Puerto Rico added as annotations at the bottom. Each state is colored based on population in 2014, using shapefiles downloaded from gadm.org. The "states.shp" set of files is available on our data page.
polyg_20.ncl: This script shows how to draw a larger map with a smaller map as an annotation. Each map is customized by drawing various outlines, turning tickmarks on/off, and adding text strings and markers.

The USA_adm2.shp shapefile downloaded from gadm.org is used to specifically draw the counties of Colorado only, while the USA_adm1.shp shapefile is used to draw the remaining state outlines.

polyg_21.ncl: This script shows how to draw large, smooth, hollow circles on a plot. If you try to draw large circles using gsn_add_polymarker and a marker index of 4, they will look blocky.

The first plot draws a single circle on a map using nggcog to create the circle and gsn_add_polyline to add the circle to the map. A hollow circle using marker index 4 and gsn_add_polymarker and is also drawn for comparison.

The second plot draws circles and ellipses using an unadvertised function called "circle_ll" contributed by Arindam Chakraborty. The circles are added to the plot using gsn_add_polyline.

See example poly_22.ncl for another way to draw smoother hollow circles, using NCL's font tables.

polyg_22.ncl: This script shows another way to draw large, smooth, hollow circles on a plot, using hollow circle markers defined in NCL's font tables #19 and #37.

See example poly_21.ncl for an example of how to create your own hollow circles.

table_6.ncl: This example draws a table of filled triangles. The triangles are filled based on a range of values.

The gsn_csm_blank_plot function was used to create a blank plot, and the filled triangles were added using gsn_add_polygon and gsn_add_polyline.

table_8.ncl: This example is similar to table_6.ncl, except it draws four triangles per each quadrant. The bottom triangles represent the minimum of the values, the top triangles represent the maximum of the values, the left triangles represent the average, and the right triangles the standard deviation. Each set of triangles has their own color map, illustrated by the four labelbars.

The second image is from running the script with nvars=5, nmodels=5, and ADD_TEXT=True. This is just for debugging purposes, so you can see what each triangle value is equal to.

Dummy data is used, which is generated by the script.

polyg_23.ncl: This script shows how to add arrows to a plot, using an "arrow" function contributed by Arindam Chakraborty.

This function uses gsn_add_polyline under the hood, which means you can use gsLineXXXX resources to customize the arrows.