(prompt "\nEarth Model in ACAD - EARTH_MODEL.LSP")
(prompt "\nCommands:  VIEW_RANGE ")
(prompt "\nFunctions: (SETUP_EARTH) (SPLIT_VPORTS) (HEIGHT_INPUT)")
(prompt "\nFunctions: ")
(prompt "\n           ")
;----------------------------------------------------------
;;;How to measure the distance to the Horizon
;;;By Takaya Iwamoto   May 10, 2005
;;;********************************************************
;;;	Copyright JUly 2006	Takaya Iwamoto
;;;********************************************************
;;;EARTH_MODEL.LSP
;;;
;;;
;;;TEST
;;;main trial routine
(defun c:View_range()
	(setup_Earth)		;setup parameters
	(split_vports)		;SPlit into 3 view ports
	(height_input)		;Input the viewer's elevation
	(reset_sysvar)
);TEST
;;;
;;;SETUP_EARTH
;;;setup for EARTH Model for estimating distance to the Horizon
(defun setup_EARTH()
	(setup_sysvar)
	(setvar "PDMODE" 32)
	(command "_.viewres" "_Y" 20000)	;;;Maximum resolution setting for ACAD
	(setq sun_rad 6.95e5 moon_rad 1.734e3 Earth_rad 6.378e3
		sun_dist 1.496e8 moon_dist 3.844e5
		moon_ctr '(0 0 0) sun_ctr (list sun_dist 0 0)
		earth_ctr (list 0  0) building_height 0.381	;;in kilometer
		building_base (list 0 Earth_rad)
		obs_point (list 0 (+ Earth_rad 1.0))
		Other_side (list 0 (- Earth_rad))
	)
	(make_pt "0" 5 earth_ctr)
	(mAKE_PT "0" 1 building_base)
		(setq base_pt (entlast))
	(make_circle_1 "0" 5 earth_ctr earth_rad)
		(setq earth (entlast))
	(make_line_1 "0" 8 other_side building_base)
	(make_line_1 "0" 8 building_base obs_point)
		(setq viewer (entlast))
	
	(command "_.zoom" "_EXTENT" )
	;(command "_.shade")
	(command "_.regen")
	
);SETUP_EARTH
;;;
;;;
(defun split_vports()
	;;;split into 3 view ports
	(command "_.vports" 3 "_Above")
	
	(setvar "CVPORT" 2)			;;;Building_base
	(command "_.zoom" "_Object" base_pt "")
	(command "_.regen")

	(setvar "CVPORT" 3)
	(command "_.zoom" "_Object" Earth "")	;;;Whole Earth
	(command "_.regen")
	(TEXTDISPLAY "Earth"   '(-2900. -500)    1700.   0.  )

	(setvar "CVPORT" 4)			;;;Building_base
	(command "_.zoom" "_Object" viewer "")
	(command "_.regen")

	
)
;;;
;;;HEIGTH_INPUT
;;;Input the elevation of the viewer in meter instead of kilometer
(defun height_input( )
	
	(setq height (getreal "\nInput the elevation of the viewer in meter (def=381)") )
	(if (= height nil) (setq height 381.))
	(setq obs_pnt (list 0 (+ earth_rad (/ height 1000.))) )
	(make_pt "0" 2 obs_pnt)
	(make_line_1 "0" 1 building_base obs_pnt)
	;;;compute the exact values and output it to the text window
	(setq exact (sqrt (*  (/ height 1000.) (+ (/ height 1000.) (* 2. Earth_rad))) ))
	(setq result (strcat "view range is: " (rtos exact 2 3) " kilometer") )
	(princ result)(terpri)
	
	
);HEIGHT_INPUT
;;;
(princ)