After using the Nested Grid Map with the Sopwith Pup, we realized that a simple Nested Grid Map does not give enough detail at low level. For example, despite what the FSX textures show, you cannot see rows of crops from high altitude - they are only 1 to 3 feet apart. We were able to create some decent looking 512x512 textures using with 8 pixels per row, which means that each texture will have only 64 rows. To put this in perspective, assume that you use a quarter section (1/2 mile square = 2,640 ft square). If you assume that crop rows are 1.5 feet apart, you need about 1600 rows (excluding border areas). Thus, you would need to tile your 512x512 texture about 25 times. This yields a resolution of about 2.25 inches per pixel, which should be a good resolution, especially from a moving airplane.

We decided to use typical Midwest farmland as our reference. This kind of farmland is familiar to pilots who train in the Midwest. For example, we generally use the center of a section as a reference for "turns around a point".

The basic building block for Midwest farmland is a section, which is 1 mile square and contains 640 acres. Around the perimeter of each section is a road, which can be part of a long paved road or a local dirt road. Within the section, the land is generally divided into 1/4 sections (1/2 mile square). Farmers typically plant different crops in each 1/4 section.

Thus, we decided to use the 1/4 section as our basic building block. This example shows how a 1/4 section might appear at low level, including basic trees. Note that you do not start seeing the individual rows until you are fairly close to the ground. Visibility is better when the crops are higher and the sun is at an angle that generates shadows between the rows.

Using 1/4 section of as our building block and using 27X27 grids increased by multiples of 3 means that we have to switch to a 3X (or greater) Nested Scrolling Grid. A simple Nested Scrolling Grid simply does not have the required range.

Here is our current classification of grids:

Here is an example of a Double Nested Grid Map where the textures have not yet been coordinated. This includes Grids 4-6. Here is an example with coordinated textures.

The Grid 5 textures are constructed using squares of 3X3 Grid 4 textures. To allow the 27X27 Grid 4 to seamlessly scroll, the maximum number of Grid 5 arrangements is limited to 81 - but we only need to use a few to achieve a random appearance. The Grid 6 textures are constructed using squares of 3X3 Grid 5 textures so that each Grid 6 texture is effectively a 9x9 grid of 1/4 sections.

Here is an example showing computer-generated farm textures. Our challenge is to smoothly transition to the close-up version of this farmland shown above.

Selecting the best Grid Size involved consideration of several factors. A primary factor is the size multiplier - i.e the increase in the size of the squares in each grid. We considered multipliers of 2X, 3X and 4X. The 2X option did not generate sufficient visibility range even when using a Double Nested Grid Map and the 4X option generated too much. The 3X option seemed the best choice.

As the Notes to the table indicate, the use of a 1/2 mile grid and 3X scaling had a several unexpected benefits. First, Grid5 has a size of 2414m, which is roughly 2X the size of the standard FSX texture. This should facilitate the use of FSX textures. Second, Digital Elevation Maps (DEMs) are typically available as 10m, 30m and 90m grids. In our system, Grid0 has a size of 9.93m, Grid1 has a size of 29.8m and Grid2 has a size of 89.41m. This should facilitate the use of DEM information with our grid if we ever decide to add height information.

This choice dictated the minimum number of squares in each grid. This is because you have to scroll enough squares to equal the size of a single square in the next larger grid. And, for appearance, you want to scroll only about 1/4 to 1/3 of the visible grid. This meant that, if scrolling 3 squares, you would want to have at least 9 squares in any direction - or a grid with 18X18 squares. The smaller, the better, because a smaller grid means fewer squares to draw. However, the grid has to be large enough so that the largest grid results in the target visibility of at least 50 miles. An 18X18 grid is too small. The smallest grid size that will generate this result is a 23X23 grid. We decided to use a 27X27 grid.