Using GEOTIFF Imagery

Introduction

This exercise assumes that you have completed the previous exercise and that Scenome is still open to the database created in the previous exercise. If you have not yet completed the previous exercise, please do so now.

These tutorials are very detailed; containing step-by-step instructions and conceptual explanations.

This is a picture of the worksheet.

Background

The imagery available for download was created from down-sampled, 1-meter MrSID imagery files. This imagery is available for download from Montana NRIS. The next exercises features MrSID imagery and shows you how to decompose MrSID files into GEOTIFF tiles.

Introduction

Terrain generation requires two types of imagery:

Elevation imagery, such as the information contained in a .DEM or .TIF file, contains data representing the height or elevation of a specific landform. These files store elevation for an area of the earth defined in degrees or parts of degrees. A DEM file typically contains elevation information for an entire degree while an TIF file contains elevation information for an entire degree or a smaller region. There are many sources of elevation imagery and many formats. Scenome uses DEM or TIF. DEM elevation data does not contain a specific map projection; TIF files are often projected in Universal Transverse Mercator.

Texture imagery contains data representing the actual appearance of the landform, often referred to as satellite or aerial imagery or photographs. In this case, texture imagery is different from a bitmap because texture imagery for terrain generation is geographically referenced. This means it contains a photograph of a specific area of the earth and usually requires a very specific map projection such as Lambert Conformal Conic or Universal Transverse Mercator. There are many forms of texture imagery; Scenome only uses GEOTIFFs.

Elevation Imagery

There are many sources of elevation imagery. The United States Government provides imagery free-of-charge for the entire United States, including Alaska and Hawaii. DEM elevation imagery is available for free download from many places.

The second type of imagery commonly used is GEOTIFF imagery. You can download GEOTIFF data from http://www.gisdatadepot.com/ or purchase the imagery for a very low cost. GEOTIFF elevation imagery contains a lot more information than DEM elevation imagery.

Best Practice

Scenome uses GEOTIFF imagery to generate texture maps for terrain. For best performance and best practice, you need two separate sets of GEOTIFF imagery for the terrain process: the first set of images uses very low resolution and is applied to the geocell preview. The second set can be very high resolution and must be subdivided into tiles around 1000×1000 pixels or smaller. Subdivision is not a requirement but it makes the texture generation process much faster and much less memory intensive.

Consider the following texture set examples:

1. Original Texture Set

The original texture set [from a provider of satellite or aerial imagery] consists of 65 GEOTIFF files that are 12012×12012 pixels but some are 12012×18018. This image set requires about 30GB of disk space. Some of the textures are larger than 600MB each. This is the input set that will be used to create the low resolution set and high resolution set. These are stock GEOTIFF images supplied by imagery providers or government agencies. Using these images at their current size is impractical and wasteful on most computer hardware.

2. Low Resolution Texture Set

The low resolution texture set consists of 65 GEOTIFF files that have been resampled to 256×256 pixels or smaller. These images are used to generate texturing for the geocell preview. Using low resolution imagery greatly speeds up texture generation on the preview mesh. Scenome triggers command line utilities that use the original texture set to create the low resolution texture set.

3. High Resolution Texture Set Example

The high resolution texture set consists of about 10,000 GEOTIFF files with a pixel resolution of 1000×1000 pixels. Subdividing each original texture into 1000×1000 pixel images creates around 144 images from each large original texture. [Remember each image in the input texture set is 12012×12012 pixels. Subdividing the original texture set produces 144 tiles.] These images are used to generate texturing for the final terrain. Scenome triggers command line utilities that use the original texture set to create the high resolution texture set.

Review

Elevation Imagery

Download DEM or SDTS imagery from the following sources:

Texture Imagery

Texture imagery is more difficult to obtain but most US states have some data available for free online.

Finished Version

There is no finished version of this model available.