We hope that most of the web site is straight forward but the Projection Explorer is a bit complicated and currently is one of a kind.
The projection explorer is not quite finished!
The Projection Explorer displays virtually all of the spatial projections available with the PROJ library. It also shows the optimal bounds for a given projection method and it's settings and clips the spatial data to fit within the bounds. This keeps the projected data from having invalid spatial data such as overlaps or "flips" where the spatial data is projected upside down, often because it on the other side of the earth from where we are viewing.
To use the Projection Explorer, select a Method from the popup menu on the left. This will change the projection method and then display a view of the earth and it's contents and countries in the panel on the right. It may take a moment as the web site has to communicate with another server to obtain, and, if needed, create the files that are used for the projection. Changing the Type of projection will give you a new set of projection methods to explore. The explorer uses grids to display the data and you can use a finer grid by changing the Grid Spacing. The software supports grids of 10, 5, 2, and 1 degree spacing but only 10 and 5 are available on the web site because of the time it takes to produce the higher resolution grids. The Max Distance Distortion entry allows you to change the maximum distance that is allowed for the projected spatial Next to the Solution label, you will see the type of solution that we are using to create the optimized bounds for the selected projection method. You may also see notes about the method. Below the solution label, you will see additional parameters that are supported by the method and the Explorer. Don't forget to click on the Description link next to the Type of method to go to the PROJ web site to learn more about the selected projection method.
In the upper left window you will see a view of the earth and the current boundary that limits the data in the map view. This boundary is used as a starting point to find the optimized bounds and you can change it by dragging the sides of the box. You can also move the arrow at the top of the box to move the longitude of origin (the longitude at the middle of the map). My favorite feature is the Go button which will animate changing the Longitude of Origin making the map spin. The Link button will show you the grid that is being used to project the data and is especially useful if you wish to use the grids to program your own web maps.
In the upper-left corner of the map view, you'll see a list of layers that are available for the map. Turn these on and off to see the various types of data available. The Projection Bounds shows the boundary that was used to crop the data to the optimized bounds. The Distortion layers show the various types of distortion that are measured by the Explorer. Area distortion is how well each cell in the grid represents the area that is actually in the same grid cell on the earth. Parallel Distortion is the amount of distortion along the parallel lines or lines of constant latitude. These lines are parallel with the equator. The Meridian Distortion shows the amount of distortion along the meridians or lines of longitude. Lines of longitude go from the north pole to the south pole. The Shape Distortion layer displays how close the angles at each corner of the grid match 90 degrees. These are colonized and a legend showing the amount of distortion for each type is available in the lower-right of the map view.