Introduction

To examine (mathematically) what two grids have in common in terms of values. Sometimes it gives valuable information to see the extent of the area where two grids have values in common and the regions where there are no common values.

The cross plot menu is found under grid > Advanced Display > Crossplot. It is a very technical menu, but should be easy to understand for the experienced Geocap user.

Features of cross plot between two datasets

Used for grids that have values in common
Display windows can be organized in tiles
Cross plot shows regional coinciding of values
Backward cross plotting of areas in cross plot

On this page:

Cross plot algorithm

The cross plot is 2 dimensional. The X axis is the data range of the first grid. The Y axis is the data range of the second grid. Geocap runs through all the grid nodes of grid 1, and check it with the same nodes value in the second grid. For instance if the first node of Grid 1 has a value of 2310 and that same node in grid 2 has the value 4250, then the point for that node is plotted as (x=2310, y=4250, z=2310 or z=4250). The z value is either the 1. or 2. scalar (grid) value.

Cross plot menu

Tiling cross plot display windows

When displaying cross plots the user can choose in which graphical window to make the plot. It gives a good overview to create three or four window displays and tile them all together. The only thing to watch is that when displaying the cross plots, be sure that the activated window is the same as the window number in the menu. If not, click Use current for the result window.

As an example for this documentation, a porosity and a net-to-gross grids were analyzed to see how the values match. The porosity grid is mapped in window 1 at upper left, the net-to-gross grid displays in upper right. The two cross plots are displayed in the lower left and lower right windows. They show the same region where values coincide, but the colors are from the first grid (scalar) and second grid respectively.

Tiling the cross plot displays

Practical work

The theme is divided into six sections which represents working steps in cross plot generation.

Create or set 3 different windows for tiling and selection

Here the user can create three windows to use in the process. Typically, the user will want to use the first window to look at grid 1, the second window for grid 2, and the third window to look at the cross plot. Note that the Use current window button sets the first/second/result window to be whichever window is active. If the user is familiar with graphical windows and tiling, you can ignore this section, and just create and tile windows as you need them.

Second grid dataset for cross plot

Here the user defines the second grid surface. The surface will be displayed in the active graphics window if the Display button is pressed. The first grid surface is to be identified in the top white box in the mapping dialogue called Input dataset for mapping.

Point display

Here the user can set the point type to be used in the cross plot to be created in the next section.

Create and display cross plot

This is where the cross plot is generated. Note that the color of each point can either be the node value of the first or the second grid (=1st or 2nd scalar). Note that it is possible to restrict the cross plot analysis to an area inside a closed polygon, or along a line (to be digitized in the next section). Use the Draw Frame and Draw Axis buttons to get axis along your cross plot. The cross plot is best viewed in 2D mode as it is a flat representation.

Quick digitize

Use this section to digitize a polygon or line in the result window to restrict your cross plot. Remember that polygons have to be closed, i.e. use the Connect to start before using the Stop button.

Select (digitize) from cross plot (result window) and display in 1. or 2. window

This option gives the user a way of digitizing a polygon in the cross plot and then afterwards displaying the grid nodes selected in the first or second grid surface, dependent on which one is activated.

Digitized area of the cross plot

The region above was digitized to see where the corresponding values in the map are located.

Location of values within digitized area in the cross plot

The location follows the contours of the map which was expected because the the selected region represents certain values in the contour band.

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