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Appendix I / Utilities / Digital Maps
_ _
22.7.2 Map Square Selection
MAPIO opens the map square containing the current geodetic position of
(p), and thereafter, as directed by MAPTHO.
(4)
(3)
MAPIO
(i,j)
22.7-2
( , λ ) µ
λqp
p
P
P
100 m
(1)
(2)
100 m
Figure 22-9 : Map Height Ordinates
µqp
Equation 22.7-2
Equi-distant 100 m map height ordinates are loaded into the working map
array. In Figure 22-9 map square (1) is loaded first containing point (p),
map squares being identified by their bottom-left hand corner (i,j).
22.7.3 Map Data Interpolation
MAPTHO takes the position (p), and returns the height of the ground at
point (f) directly below, together with the E/W and N/S ground slopes there.
⎛
MAPTHO ⎜ µ p , λp
, P
⎜
⎝
ZG
f , p
ZG
p
∂ Pf
,
,
∂ µ
p
ZG
p
∂ Pf
,
,
∂ λ
p
⎞
⎟
⎟
⎠
Equation 22.7-3
16 map heights are extracted in the square {(i,j):i,j = [-1(1)2]}, starting with
those which fall in the map square (1) containing (p). Nearby maps are
accessed to collect ordinates from squares (2) to (4) by calling MAPIO.
When the ordinates defining the 300 m patch have been extracted a 3 rd order
bi-cubic spline is fitted by SURF and interpolation used to extract the map
height and ground slopes at (p) in MAPKAL. The map data extracted
contain random triangulation and digitisation errors that differ between map
sources and digitisation techniques used.