Study on the Key Technique of the Hill Shading Virtual Roaming System Based on XNA - Advances in Image and Graphics Technologies - page 231

in Section 4, the method of shading terrain block model in XNA is explicated; in
Section 5, the shading effects of the roaming system prototype are offered and the
conclusions are discussed in the last Section.

2 LOD Structure

It cannot do without DEM data to create hill shading effect [2]. The roaming system
uses DEM data to downloade geographic information from the platform of common
services, such as CGIAR-CSI, NASA, etc. These data sets include the DEM data of
six kinds of resolution. They are SRTM 90m V4.1, ASTER GDEM and re-sampled
SRTM data to 250m, 500m and 1km.
These DEM data take up a lot of storage space. The size of SRTM 500m DEM
ASCII file format data after decompression is more than 10G. These data can't be pre-
loaded during the system initialization in the main memory. The LOD technique, whis
doesn't represent the main focus of this essay, is commonly used to build a large-scale
terrain roaming system in engineering. Only the method described here is used in the
prototype. The prototype designed to use the static LOD. Select the LOD level based
on the distance between the viewpoint and terrain surfaces. The DEM data of each
level was split into a group of small terrain block files. Each block file has 301×301
height data. is the choice of this size was decided to keep the accuracy of the original
DEM data, yet without the requirement for data interpolation. The data exchange
format of terrain block file is ArcInfo ASCII(described in reference[3]). The terrain
block files at the same level were named by the unified coding.
It was designed to add three levels LOD data to further simplify the DEM data.
These data were re-sampled based on SRTM 1km ASCII data, that signed in the
following table(Table 1) by marked * in LOD level column. The resolution of other
levels remains.

Table 1. Resolution of DEM data at each LOD level

LOD level resolution of DEM ( ° )

Data type

1* SRTM 12km 0.099999999996

2* SRTM 6km 0.049999999998

3* SRTM 2km 0.016666666666

4 SRTM 1km 0.008333333333

5 SRTM 500m 0.004166666666

6 SRTM 250m 0.002083333330

7 SRTM 90m DEM 0.000833333333

8 ASTER GDEM 0.000277777778

3 Terrain Block Model

3.1 Defining the Vertices

The prototype system uses color shading. While creating hill shading effect requires
the use of lighting model in XNA. So the vertex structure of terrain block should also