70
© 2016 Open Geospatial Consortium
Requirement 40
http:opengis.netspecCDB1.0cor egeospecific-storage
Like all of the CDB Tiled datasets, geospecific models SHALL be stored in the \CDB\Tiles\ directory. As a result, client-devices can reference each model with a unique
directory path and a unique file name which is derived from the model’s unique position, level-of-detail, and its feature code
See section 6.4.7.2, Volume 10 OGC CDB Implementation Guidance for more implementation guidance on this topic.
3.3.6 Organizing Models into Levels of Detail
Requirement 41
http:opengis.netspecCDB1.0cor elod-organization-resolution
The geometry, texture, and signature datasets of 3D models SHALL be organized into levels of details LOD based on their resolutions.
The expression of the model resolution depends on the dataset; the resolution of the model geometry is called its Significant Size SS; the texture resolution is expressed by its Texel Size
TS; and for the radar signature of a model, its resolution is simply its size and is measured by the diameter of its Bounding Sphere BSD.
The lower bounds LB of SS, TS, and BSD for a given LOD can be expressed by the following set of equations.
71
© 2016 Open Geospatial Consortium
In all three equations, the value 111319 represents the approximate length in meters of an arc of one degree at the equator
22
. For convenience, the following table gives the CDB LOD associated with these three measures
of the resolution of a model. Note that all values are expressed in meters using a scientific notation with 6 decimals.
Table 3-1: CDB LOD vs. Model Resolution ModelGeometry
ModelTexture ModelSignature
CDB LOD Significant Size
Texel Size Bounding Sphere
-10 SS 5.565950 × 10
+4
TS 6.957438 × 10
+3
BSD 4.452760 × 10
+5
-9 SS 2.782975 × 10
+4
TS 3.478719 × 10
+3
BSD 2.226380 × 10
+5
-8 SS 1.391488 × 10
+4
TS 1.739359 × 10
+3
BSD 1.113190 × 10
+5
-7 SS 6.957438 × 10
+3
TS 8.696797 × 10
+2
BSD 5.565950 × 10
+4
-6 SS 3.478719 × 10
+3
TS 4.348398 × 10
+2
BSD 2.782975 × 10
+4
-5 SS 1.739359 × 10
+3
TS 2.174199 × 10
+2
BSD 1.391488 × 10
+4
-4 SS 8.696797 × 10
+2
TS 1.087100 × 10
+2
BSD 6.957438 × 10
+3
-3 SS 4.348398 × 10
+2
TS 5.435498 × 10
+1
BSD 3.478719 × 10
+3
-2 SS 2.174199 × 10
+2
TS 2.717749 × 10
+1
BSD 1.739359 × 10
+3
-1 SS 1.087100 × 10
+2
TS 1.358875 × 10
+1
BSD 8.696797 × 10
+2
SS 5.435498 × 10
+1
TS 6.794373 × 10
+0
BSD 4.348398 × 10
+2
1 SS 2.717749 × 10
+1
TS 3.397186 × 10
+0
BSD 2.174199 × 10
+2
2 SS 1.358875 × 10
+1
TS 1.698593 × 10
+0
BSD 1.087100 × 10
+2
3 SS 6.794373 × 10
+0
TS 8.492966 × 10
−1
BSD 5.435498 × 10
+1
4 SS 3.397186 × 10
+0
TS 4.246483 × 10
−1
BSD 2.717749 × 10
+1
22
The actual equation to obtain the values of 111319 m is L=a×PI180° where “a” is the length of the major semi- axis of the WGS-84 ellipsoid; “a” is also known as the equatorial radius.
72
© 2016 Open Geospatial Consortium 5
SS 1.698593 × 10
+0
TS 2.123241 × 10
−1
BSD 1.358875 × 10
+1
6 SS 8.492966 × 10
−1
TS 1.061621 × 10
−1
BSD 6.794373 × 10
+0
7 SS 4.246483 × 10
−1
TS 5.308104 × 10
−2
BSD 3.397186 × 10
+0
8 SS 2.123241 × 10
−1
TS 2.654052 × 10
−2
BSD 1.698593 × 10
+0
9 SS 1.061621 × 10
−1
TS 1.327026 × 10
−2
BSD 8.492966 × 10
−1
10 SS 5.308104 × 10
−2
TS 6.635129 × 10
−3
BSD 4.246483 × 10
−1
11 SS 2.654052 × 10
−2
TS 3.317565 × 10
−3
BSD 2.123241 × 10
−1
12 SS 1.327026 × 10
−2
TS 1.658782 × 10
−3
BSD 1.061621 × 10
−1
13 SS 6.635129 × 10
−3
TS 8.293912 × 10
−4
BSD 5.308104 × 10
−2
14 SS 3.317565 × 10
−3
TS 4.146956 × 10
−4
BSD 2.654052 × 10
−2
15 SS 1.658782 × 10
−3
TS 2.073478 × 10
−4
BSD 1.327026 × 10
−2
16 SS 8.293912 × 10
−4
TS 1.036739 × 10
−4
BSD 6.635129 × 10
−3
17 SS 4.146956 × 10
−4
TS 5.183695 × 10
−5
BSD 3.317565 × 10
−3
18 SS 2.073478 × 10
−4
TS 2.591847 × 10
−5
BSD 1.658782 × 10
−3
19 SS 1.036739 × 10
−4
TS 1.295924 × 10
−5
BSD 8.293912 × 10
−4
20 SS 5.183695 × 10
−5
TS 6.479619 × 10
−6
BSD 4.146956 × 10
−4
21 SS 2.591847 × 10
−5
TS 3.239809 × 10
−6
BSD 2.073478 × 10
−4
22 SS 1.295924 × 10
−5
TS 1.629905 × 10
−6
BSD 1.036739 × 10
−4
23 SS 0
TS 0 BSD 0
When using the table to perform a lookup, first compute the value of SS, TS, or BSD, then scan through the lines of the table starting at the top with LOD −10; when the computed value is
larger than the lower bound of the LOD, select that LOD. Since the values of SS, TS, and BSD are, by definition, always positive, the search for a LOD will always be successful; in the worst
case, the search will end with the last line of the table.
73
© 2016 Open Geospatial Consortium
3.3.7 Organizing Models into Datasets
