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Tunnicliff 1791, ngr overlay

Simple mathematics has been used to crudely superimpose the national grid on an old map. The difference between the map projections used by the NGR system and by the old map's maker is ignored; for a county area at this scale it is not important. 'Centres' of the the old and the Ordnance Survey maps of the county were superimposed at an average position of twenty or so prominent towns; the orientations and scales were matched by averaging the directions and distances of these places from the centres on each map; it is then possible to draw the NGR from the modern map upon the image of the old. The calculations were done by a computer using a program, written by MN, called OLDMAPS.exe; fuller notes about the process are provided in:-

Input File

Input to the program is file:-
 
TUN1PTS.txt
which has readings of positions of recognisable places on the old map, using (x,y) coordinates on the old map measured in cm from an origin at the intersection of the bottom and left map borders. (The software accepts coordinates in pixels from an origin top left, measurements made on the archived .tif image, this is far easier. Because of a software glitch the pixels are record in 100s, eg 24.15 is 2415 pixels.)
 

@t Tunnicliff 1791

@pl Alton @ngr SU717396 @xy (42.27,40.70)
@pl Andover @ngr SU365458 @xy (21.76,44.68)
@pl Burghclere @ngr SU469610 @xy (28.72,52.52)
@pl Buriton @ngr SU740200 @xy (43.09,29.44)
@pl Christchurch @ngr SZ159925 @xy (9.73,14.27)
@pl Fareham @ngr SU575061 @xy (34.46,21.68)
@pl Fordingbridge @ngr SU150145 @xy (8.62,27.32)
@pl Grately @ngr SU275420 @xy (16.72,42.25)
@pl Havant @ngr SU717062 @xy (42.10,21.56)
@pl Kingsclere @ngr SU525586 @xy (31.77,53.13)
@pl Lymington @ngr SZ322954 @xy (18.55,16.01)
@pl Nether Wallop @ngr SU304364 @xy (18.05,39.23)
@pl Overton @ngr SU514500 @xy (31.15,47.79)
@pl Petersfield @ngr SU744235 @xy (43.18,31.07)
@pl Romsey @ngr SU351212 @xy (21.42,29.92)
@pl Stockbridge @ngr SU355351 @xy (21.46,38.50)
@pl Tangley @ngr SU334524 @xy (20.13,49.32)
@pl Whitchurch @ngr SU460478 @xy (27.64,46.53)
@pl Winchester @ngr SU478293 @xy (28.57,35.03)

@w 56.01 @h 60.64
@sc 9.52 @m 10


Output File

OLDMAPS.exe outputs to:-
 
TUN1NGR.txt
The file was designed for use by JandMN and is not terrifically friendly, sorry! The essential data is given here; some data is for information rather than use.

Centroids of points

The centroids of the towns in (x,y) space and in ngr space are reported:-
 
(x,y) centroid is (27.69,36.44)
NGR centroid is SU464314; numerical ngr 446.47,131.42

Map scale

the map's scale is calculated by comparing the distances of the towns from the centroids.
 
scale = 1.75 (Alton)
scale = 1.72 (Andover)
scale = 1.84 (Burghclere)
scale = 1.76 (Buriton)
scale = 1.73 (Christchurch)
scale = 1.70 (Fareham)
scale = 1.69 (Fordingbridge)
scale = 1.75 (Grately)
scale = 1.72 (Havant)
scale = 1.62 (Kingsclere)
scale = 1.73 (Lymington)
scale = 1.68 (Nether Wallop)
scale = 1.66 (Odiham)
scale = 1.62 (Overton)
scale = 1.77 (Petersfield)
scale = 1.69 (Romsey)
scale = 1.76 (Stockbridge)
scale = 1.66 (Tangley)
and averaged, and rounded off so as not to mislead the user with false accuracy:-
 
map scale 1 to 171440 (range 162087 to 183669)
about 1 to 171000

Map heading

The map's heading is calculated by comparing the headings of the towns from the centroids, and averaging:-
 
NGR north on map is 1.1 degrees from map north

Check points

The position of the ngr grid on the old map is now 'known' by the software. The location of the towns on the map and in ngr space can be compared, and an error calculated, in Km.
         
place   true ngr map ngr error, Km
         
1 Alton   SU717396 SU713391 0.64
2 Andover   SU365458 SU360453 0.71
3 Burghclere   SU469610 SU477590 2.15
4 Buriton   SU740200 SU730199 1.00
5 Christchurch   SZ159925 SZ163928 0.50
6 Fareham   SU575061 SU585063 1.02
7 Fordingbridge   SU150145 SU140151 1.17
8 Grately   SU275420 SU274410 1.00
9 Havant   SU717062 SU716063 0.14
10 Kingsclere   SU525586 SU529601 1.55
11 Lymington   SZ322954 SZ314960 1.00
12 Nether Wallop   SU304364 SU298358 0.85
13 Odiham   SU740509 SU747516 0.99
14 Overton   SU514500 SU520509 1.08
15 Petersfield   SU744235 SU731226 1.58
16 Romsey   SU351212 SU359200 1.44
17 Stockbridge   SU355351 SU357347 0.45
18 Tangley   SU334524 SU331532 0.85
19 Whitchurch   SU460478 SU460487 0.90
20 Winchester   SU478293 SU480290 0.36

Map sheet corners

The grid references of the maps corners is given (the corners from which measurements were made):-
 
bottom left SZ001680
top left ST982720
top right SU942737
bottom right SZ961698
This could be useful as an indexing tool. The map's coverage could be drawn in a GIS system. We have not done this for a set of county maps which too fairly closely.

Grid lines

The software provides marker points by which to drawn ngr grid lines on the original map (on an overlay! or on the scanned map image). These are: x positions on bottom and top edges, y positions on left and right edges.

easting grid lines


northing grid lines

     
Eing bottom edge top edge
     
400 -0.10 1.03
410 5.74 6.86
420 11.57 12.70
430 17.40 18.53
440 23.24 24.36
450 29.07 30.20
460 34.91 36.03
470 40.74 41.87
480 46.57 47.70
490 52.41 53.53
500 58.24 59.37
     
     
Ning left edge right edge
     
70 1.13 0.09
80 6.96 5.92
90 12.80 11.76
100 18.63 17.59
110 24.46 23.42
120 30.30 29.26
130 36.13 35.09
140 41.97 40.93
150 47.80 46.76
160 53.63 52.59
170 59.47 58.43
180 65.30 64.26

Scale line

The scale line on the map is used to calculate what it says the map's scale is:-
 
scale line length 9.52 cm
representing 10 miles
scale 1 to 169049
Separate software, DISTTAB.exe, does a more thorough job of calculating map scale.

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