Precision optical square instrument

A precision optical square instrument for accurately determining lines and planes respectively perpendicular to reference lines and planes comprising, a substantially hollow housing containing first and second reflectors arranged at different vertical positions for viewing first and second marker positions outside the housing which are substantially at right angles to each other. The images of the two marker positions are projected onto a third reflector which is viewed by a user, such that the user can determine when the two marker positions are located perpendicularly, by the alignment of the images viewed in the third reflector.

The present invention relates to a precision optical square instrument for 
accurately determining lines and planes respectively perpendicular to 
reference lines and planes. 
One known method of setting out a building site after trenches have been 
excavated and foundations laid, is to establish the lines of the eventual 
building walls above ground level by using string stretched between 
markers known in the trade as profiles. The locations of the eventual 
building corners are then transferred to the foundation by dropping a 
vertical line to the trenches This method is slow and due to its nature is 
open to inaccuracies. 
In the field of surveying, optical square instruments are known, however, 
one problem is that they are confined to use in open, unobstructed spaces 
and are either hand held or held on the end of a ranging rod thereby 
reducing their accuracy An optical square instrument comprising two 
telescopes mounted perpendicularly to one another and mounted on a tripod 
is also known, however, this instrument has the disadvantage that it is 
expensive and requires considerable space and skill to set up. 
An object of the present invention is to provide a precision optical square 
instrument which mitigates the above problems. 
According to the present invention there is provided a precision optical 
square instrument for accurately determining lines and planes respectively 
perpendicular to reference lines and planes comprising, a substantially 
hollow housing containing a first and second reflecting means, arranged at 
different vertical positions for viewing first and second marker positions 
outside the housing which are substantially at right angles to each other, 
and projecting images of the two marker positions onto third reflecting 
means which is viewed by a user, such that the user can determine when the 
two marker positions are located perpendicularly, by the alignment of the 
images viewed in the third reflecting means.

Referring now to the drawings wherein similar numerals have been used to 
indicate like parts there is shown therein a precision optical square 
instrument generally indicated at 10 according to the invention. The 
optical square instrument 10 comprises a substantially hollow housing 11 
which is square when viewed in plan, and comprises a body including 
sidewalls 12,13,14 and 15, a base 16, together with a top 17. The 
sidewalls 12,13 each have a perspex or glass viewing window 18,19 
respectively fixed therein with the window 19 being at a slightly higher 
position than the window 18, the reason for which is described below. The 
viewing windows are located at 90.degree. relative to each other. The top 
17 also has a viewing window 20. The underside 21 of the base 16 has two 
locating arms 22,22' arranged perpendicular to each other, with the arm 22 
also being perpendicular to the sidewall 12 and the arm 22' being 
perpendicular to the sidewall 13. A sheet of rubber or plastics 23 is 
fixed to the underside of the base 16 in the region between the locating 
arms 22,22'. 
The top 17 is fixed to the body by means of screws (not shown) which engage 
in apertures 24 in the top 17, and corresponding holes 25 at the corners 
of the housing 11. 
A first or lower mirror 30 is fixed to a base support 31 which is fixed to 
the base 16 inside the housing 11. The mirror 30 is arranged perpendicular 
to the base 16. A second or upper mirror 32 is also fixed to a base 
support 33 which is arranged to be fixed on top of the base support 31 so 
that the mirror 32 is also located perpendicular to the base 16 but 
obliquely relative to the mirror 30. A locating pin 34 projecting 
downwardly from the underside of base support 33 is received in a 
correspondingly located hole 35 in the base support 31. The base support 
31 has a screw threaded hole 36 for receiving a screw 37 the shank 38 of 
which passes through a larger hole 39 in the base support 33 of the upper 
mirror 32. The upper and lower mirrors 30,32 are fixed one on top of the 
other by means of the screw 37 at an inclusive angle A of 135 relative to 
each other. With the screw 37 loosened, the base support 33 can rotate 
very slightly on the base support 31 around the axis of the locating pin 
34, which is desirable when the instrument is being assembled so that the 
angle of the mirrors 30,32 relative to each other can be slightly adjusted 
and precisely obtained This provides an adjustment which may be desirable 
to offset deviations which may arise due to manufacturing tolerances. 
A third mirror 40 which is elongate is also located in the housing 11 and 
is mounted on a base support 41 which is fixed to the base 16 of the 
housing. The mirror 40 is inclined at an angle of 45.degree. to the base 
16. The longitudinal center line of the mirror 40 lies on a notional 
vertical plane passing through a diagonal X-Y of the housing. Also, a 
notional vertical line Z passing through the intersection of the planes of 
the lower and upper mirrors 30,32 lies in the notional vertical plane 
passing through the diagonal X-Y. Furthermore, the notional vertical line 
of intersection Z of the mirrors 30,32 is coincident with the vertical 
line of intersection 50 of the inner surfaces 42 of the locating arms 
22,22'. The mirror 40 has a pair of hair-lines 51 formed therein and 
equally spaced about the longitudinal axis of the mirror 40. The window 20 
in the top 17 of the housing is located directly over the mirror 40. In 
the construction as described, the upper mirror 32 views an image through 
the window 19 and reflects this image onto the mirror 40 which in turn 
reflects the image vertically upwards through the window 20 to be viewed 
by a user. Similarly, the lower mirror 30 views an image through the 
window 18 and reflects this image onto the mirror 40 which reflects the 
image vertically upwards through the window 20 to be viewed by a user. 
Therefore, the images viewed by a user looking vertically downwardly 
through the window 20 are the images reflected by the mirrors 30,32 which 
will be seen one on top of the other. 
Referring to FIG. 6, in use, if the instrument is to be used to set out 
blockwork, a first line 60 is established by conventional methods. A 
building block 61 is placed at a corner where it is desired to establish a 
line perpendicular to the first line 60. The longer side 62 of the block 
61 is placed along the first line 60 with the shorter side 63 of the block 
located along the eventual line 64 perpendicular to the first line 60. The 
instrument 10 is then placed on a corner of the block 61 as shown, with 
the window 18 viewing along the line 60 and the window 19 viewing in the 
direction parallel to the shorter side 63 of the block. The instrument 10 
is located on the block 61 so that the inner surface 42 of the locating 
arm 22 abuts the side 62 of the block 61 and the inner surface 42 of the 
locating arm 22' abuts the shorter side 63 of the block with the notional 
vertical line of intersection 50 of the inner surfaces 42 coinciding with 
the edge 65 of the block 61. The rubber or plastics sheet 23 on the 
underside 16 of the housing 11 provides a friction grip for the instrument 
on the block surface. 
A second block 66 is located along the first line 60 about 8 meters from 
the edge 65 of the first block 61 along the line of sight of the window 
18. A profile member 67 is secured to the first block 61 by means of a 
suitable clamp 59. The profile member 67 comprises an elongate slat having 
an outwardly projecting locating arm 68 at its lower end. The profile 
member 67 is secured to the block 66 with the locating arm 68 abutting the 
longer side 69 of the block 66. The profile member 67 is painted on the 
surface 70 with two dissimilar colors 71,72, so that when secured in place 
to the block 66, the center line 73 coincides with the outermost corner 74 
of the block 66. Also, the profile member 67 has a spirit level (not 
shown) suitably fixed thereto so that a user can determine when the 
profile member 67 is positioned vertically. 
A second profile member 67' similar to profile member 67 is located along 
the line of sight of the window 19 of the instrument and about 6 meters 
from the edge 65 of the block 61 as shown It will be noted from FIG. 6 
that profile member 67' is effectively a mirror image of profile member 
67. In FIG. 6 another block 66' has been shown to which the second profile 
member 67' is secured, however, in practice the profile member 67' may 
simply be manually held vertically if desired. 
The images of the profile members 67,67' are viewed by a user through the 
window 20 in the top 17 of the instrument 10. The image 80 of the first 
profile member 67 is reflected by the lower mirror 30 onto the mirror 40 
which is viewed by the user. The image 81 of the second profile member 67' 
is also reflected by the upper mirror 32 onto the mirror 40. The images 
80,81 as seen by the user are indicated in FIGS. 7a to 7d with the images 
in 
FIGS. 7a and 7b indicating that the profile members 67,67' lie on notional 
perpendicular lines 60,64 by the alignment of the center lines 73,73' of 
the profile members, and the images in FIGS. 7c and 7d indicating that the 
profile members are not arranged perpendicularly. The profile member 67' 
is thus moved from side to side transversely of the viewing direction of 
window 19 until the correct image is seen by the user, indicated by the 
alignment of the images as illustrated in FIGS. 7a or 7b. The 
perpendicularity of the lines 60,64 can further be checked by measuring 
the hypotenuse of the notional triangle joining blocks 61, 66 and 66'. 
The two lines 51 on the mirror 40 will also be seen when the user views the 
mirror 40 and provide a guide for the user to determine the center line of 
the mirror 40 for the coincidental alignment of the center lines of the 
profile members. 
It will be understood that in another embodiment of the invention each of 
the mirrors may be replaced by a suitable reflecting prism 
The advantage of the invention is that lines and planes can quickly be 
established which are respectively perpendicular to reference lines and 
planes. The invention is simple and inexpensive to manufacture and is 
readily used by unskilled people.