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Draw a Tk Canvas using Ordnance Survey data
this example from a Well House Consultants training course
More on this [link]

This example is described in the following article(s):
   • Travel Across Wiltshire - the game - [link]

Source code: osmap.tk Module: M200

set reftab {{0 0 SV SW SX SY SZ TV}
{100 100 SR SS ST SU TQ TR}
{100 200 SM SN SO SP TL TM}
{200 300 SH SJ SK TF TG}
{200 400 SC SD SE TA}
{100 500 NW NX NY NZ OV}
{100 600 NR NS NT NU GX}
{0 700 NL NM NN NO}
{0 800 NF NG NH NJ NK}
{0 900 NA NB NC ND}
{100 1000 HW HX HY HZ}
{300 1100 HT HU}
{400 1200 HP}}

proc convert from {
global reftab
regexp (..)(...)(...) $from all let east north
regsub "^0{1,2}" $east "" east
regsub "^0{1,2}" $north "" north
foreach row $reftab {
for {set col 2} {$col < [llength $row]} {incr col} {
        if {[lindex $row $col] == $let} {
                set nbase [lindex $row 1]
                set ebase [lindex $row 1]
                set retnorth [expr $nbase * 10 + $north]
                set reteast [expr ($ebase + 100 * ($col -2))*10 + $east]
                return "$reteast $retnorth"
return 0

proc propn {left right steps stepno} {
        return [expr int($left + ($right - $left) / $steps. * $stepno)]

proc cvt from {
global leftbase topbase
global gridwid gridhei
global canwid canhei
        set gridit [convert $from]
        set easting [expr ([lindex $gridit 0] - $leftbase) * $canwid / $gridwid ]
        set northing [expr ($topbase - [lindex $gridit 1]) * $canhei / $gridhei ]
        return "$easting $northing"

set topleft ST730950
set bottomright SU344171
set tlpoint [convert $topleft]
set brpoint [convert $bottomright]
set leftbase [lindex $tlpoint 0]
set topbase [lindex $tlpoint 1]
set gridwid [expr [lindex $brpoint 0] - [lindex $tlpoint 0]]
set gridhei [expr [lindex $tlpoint 1] - [lindex $brpoint 1]]

set canhei 700
set canwid [expr int($canhei. * $gridwid / $gridhei)]

proc plot from {
        global pixwid pixcol
        set type [lindex $from 0]
        set os 2
        switch $type {
                join {
                set start [cvt [lindex $from 1]]
                set to [cvt [lindex $from 2]]
                set steps [expr [lindex $from 3] / 3]
                .wiltshire create line [lindex $start 0] [lindex $start 1] \
                        [lindex $to 0] [lindex $to 1] \
                        -width $pixwid -fill $pixcol
                for {set k 0} {$k<=$steps} {incr k} {
                        set xc [propn [lindex $start 0] \
                                [lindex $to 0] $steps $k]
                        set yc [propn [lindex $start 1] \
                                [lindex $to 1] $steps $k]
                        set xl [expr $xc + $os]
                        set yl [expr $yc + $os]
                        set xr [expr $xc - $os]
                        set yr [expr $yc - $os]
                        .wiltshire create oval \
                                $xl $yl $xr $yr \
                                -fill black
                place {
                set gridref [lindex $from 1]
                set wording [lrange $from 2 end]
                set placement [cvt $gridref]
                .wiltshire create text \
                        [lindex $placement 0] [lindex $placement 1] \
                        -anchor w -text " $wording"}

                width {
                set pixwid [lindex $from 1] }

                color {
                set pixcol [lindex $from 1] }

canvas .wiltshire -width $canwid -height $canhei -background white
pack .wiltshire

set fhx [open tracks.txt r]

set pixwid 1
set pixcol black

while {[gets $fhx here] > -1} {
        plot $here
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