The Flight Simulator Terrain System (photoscenery and landclass)

This discussion is something that I have been promising our users for some time. Many users are already quite knowledgeable when it comes to flight simulation products and what they actually provide. However, many new users, or even experienced flyers, find the differences between photoscenery and landclass very confusing.

Since we are in the business of providing terrain scenery for Flight Simulators, this blog entry may some of you to better understand the flight simulation terrain system in less technical terms (hopefully).

MSFS Terrain System

Someday, flight simulators will be able to cover the entire earth with consistent, beautifully colored, recent, high resolution imagery. To a smaller degree, the technology is here today. And, there are several fine photoscenery products available that support both FSX and FS9. However, neither the data (imagery) nor hardware capabilities are at the point where the world, or even individual continents can be quickly developed, easily distributed and stored on a user’s computer.

When you look at the terrain scenery in Microsoft’s Flight Simulators, the world appears as one continuous skin covering the entire globe. In reality, the entire world is broken into measurable 1km x 1km square cells. So, the world is one big grid system of individually tiled images that wrap around the globe in a North/South and East/West direction. Each of these small 1km x 1km cells is assigned a particular graphic image that is to be displayed. That graphic can be unique (photoscenery) or selected from a set of available graphic images (as in landclass).

Photoscenery

In the case of photoscenery, each cell is assigned to a unique image that depicts the portion of the globe represented by the 1km large cell boundaries. How many cells does it take to cover the entire earth ? Well, the earth is broken up into 24,576 columns x 32,769 rows of these 1km square cells. So, the total number of cells for the entire globe is 805,306,368. That is a lot of individual cells that must be maintained by the flight simulator. What if you wanted to display the entire globe with photoscenery right now. Even if a global product was available, here are some rough numbers for you…

Right now in FSX, each cell is represented by a 1024x1024 pixel image (roughly 1m per pixel resolution). At this resolution, the bitmap storage requirements for each image in FSX (DXT1 format) is 699,122 bytes. In order to create unique textures for the entire globe, for a single season, you would require the following disk storage space:

805,306,368 cells x 699,122 bytes = 563,007,398,608,896 (roughly 563 terabytes of storage)

The roughly 563 terabytes would be necessary for depicting single daytime season (i.e summer). However, you need to multiply this figure by 6 to have 4 seasons depicted, plus a hard winter texture, plus a night texture. With unique 6 image versions of each cell necessary, we are talking about storage requirements in the petabyte range (I had to research that one in wikipedia).

You can see that without major changes in technology or image compression, covering the world in photoscenery is still some ways off. Also, the video card has to have enough RAM to hold the textures needed for a current scenery area when flying in a simulator. If the video card cannot hold all the images in its onboard RAM, it has to constantly swap the images in and out with the disk (paging). This will really slow down your performance.

For obvious reasons, photoscenery is a very popular addon product for flight sims. If it is well done, it provides the pinnacle in flight simulation terrain scenery. But, photoscenery products are currently restricted to smaller regional areas, like major cities of the world or very scenic regions.

Photoscenery is not the only way of depicting terrain in FSX and FS9. In fact, Microsoft does not provide any photoscenic areas in the retail FSX product. If photoscenery is not used to draw the terrain, the terrain is drawn using the landclass system (with vector graphic supplements).

Tomorrow’s blog entry will describe the MSFS landclass system and how it is used.