National Grid References

Blome's Hampshire 1673
NGRs Simple mathematics was used to crudely superimpose the national grid on Blome's map. The difference between the map projections used by the NGR system and used by Richard Blome has been ignored. '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 sca les 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 wri tten by MN, OLDMAPS.exe; fuller notes about the process are provided in the software notes, OLDMAPS.rul.

File BLM1PTS.txt has readings of positions of recognisable places on the old map, using (x,y) coordinates on the old map.
Points
file		 

@pl Alton @ngr SU718393 @xy (19.17,22.15)
@pl Andover @ngr SU365458 @xy (8.43,21.64)
@pl Basingstoke @ngr SU634527 @xy (16.60,25.03)
@pl Christ church @ngr SZ159925 @xy (3.64,4.01)
@pl Fareham @ngr SU575061 @xy (16.19,11.03)
@pl Fordingbridge @ngr SU150145 @xy (3.54,10.92)
@pl Gosporte @ngr SZ616998 @xy (16.87,8.34)
@pl Havant @ngr SU717062 @xy (20.17,11.19)
@pl Kingscler @ngr SU525586 @xy (12.95,26.38)
@pl Lymingto~ @ngr SZ322954 @xy (8.83,6.23)
@pl Alesford @ngr SU588326 @xy (15.68,19.01)
@pl Odiam @ngr SU740509 @xy (19.30,25.05)
@pl Petersfeild @ngr SU744235 @xy (20.05,17.12)
@pl Portesmouth @ngr SZ632993 @xy (17.65,8.52)
@pl Ringwood @ngr SU149054 @xy (4.14,7.98)
@pl Rumsey @ngr SU351212 @xy (9.23,14.07)
@pl Southampto~ @ngr SU417122 @xy (11.59,10.59)
@pl Stokbridg @ngr SU355351 @xy (8.78,18.04)
@sc 6.21 @m 10
@w 25.96 @h 30.23
The results from OLDMAPS.exe are given in file: BLM1NGR.txt:-
Output from OLDMAPS.exe, using BLM1PTS.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 (12.93,14.85)
     NGR centroid is SU486217; numerical ngr 448.65,121.73

Map scale

the map's scale is calculated by comparing the distances of the 
towns from the centroids:-
 
     scale = 3.03 (Alton)
     scale = 3.31 (Andover)
     scale = 3.17 (Basingstoke)
     scale = 3.07 (Christ church)
     scale = 3.58 (Fareham)
     scale = 3.38 (Fordingbridge)
     scale = 3.35 (Gosporte)
     scale = 3.43 (Havant)
     scale = 3.25 (Lymingto~)
     scale = 2.98 (Alesford)
     scale = 3.21 (Odiam)
     scale = 3.46 (Petersfeild)
     scale = 3.39 (Portesmouth)
     scale = 3.36 (Ringwood)
     scale = 2.64 (Southampto~)
     scale = 3.58 (Stokbridg)

and averaged, and rounded off so as not to mislead the user 
with false accuracy:-

     map scale 1 to 326169 (range 264012 to 358088)
     about 1 to 326000

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 -8.6 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

Alton          SU718393       SU723422       2.94
Andover        SU365458       SU374458       0.90
Basingstoke    SU634527       SU654527       2.00
Christ church  SZ159925       SZ133913       2.86
Fareham        SU575061       SU572078       1.73
Fordingbridge  SU150145       SU164136       1.66
Gosporte       SZ616998       SZ581988       3.64
Havant         SU717062       SU701063       1.60
Kingscler      SU525586       SU543589       1.82
Lymingto~      SZ322954       SZ311959       1.21
Alesford       SU588326       SU595337       1.30
Odiam          SU740509       SU741514       0.51
Petersfeild    SU744235       SU727255       2.62
Portesmouth    SZ632993       SZ607990       2.52
Ringwood       SU149054       SU169038       2.56
Rumsey         SU351212       SU363210       1.22
Southampto~    SU417122       SU422086       3.63
Stokbridg      SU355351       SU368340       1.70

                                    average  2.02

Map sheet corners

The grid references of the maps corners is given (the corners 
from which measurements were made):-
     bottom left   SY996801
     top left      SU144776
     top right     SU981649
     bottom right  SZ833674

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

Eing      bottom edge    top edge

400        0.10          -4.49
410        3.21          -1.39
420        6.31           1.71
430        9.41           4.81
440       12.51           7.91
450       15.61          11.01
460       18.71          14.12
470       21.81          17.22
480       24.91          20.32
490       28.01          23.42
500       31.12          26.52

     northing grid lines


Ning      left edge      right edge

 70       -3.16           0.79
 80       -0.06           3.89
 90        3.05           6.99
100        6.15          10.09
110        9.25          13.19
120       12.35          16.29
130       15.45          19.40
140       18.55          22.50
150       21.65          25.60
160       24.75          28.70
170       27.85          31.80
180       30.96          34.90


Scale line

The scale line on the map is used to calculate what it 
says the map's scale is:-

     scale line length 6.21 cm
     representing 10 miles

     scale 1 to 259154

Separate software, DISTTAB.exe, does a more thorough job 
of calculating map scale.


Blome's Hampshire 1673, index grid
General index
Old Hampshire Mapped
Map HMCMS:FA1991.9