It particularly relates to a plane table for map making.

Surveyors utilise a plane table for direct map making in the field, the plotted work being made while the survey proceeds, rather than being drawn later on from measurements taken. It would be desirable to provide an educational apparatus or toy plane table, to teach the principies of mapping since a conventional plane table is an expensive, precision made piece of equipment.

A surveyor's plane table has a precisely made hardwood top mounted on a rotatable mount or racer ring to the top of a tripod having independently height adjustable legs. The tripod mount provides means for adjusting the level of the plane table top which can be set using a spirit level. A compass is used to orientate the table.

In use, a plumb bob serves to centre the plane table over a datum point on the ground. An alidade is used to sight points to be mapped and for drawing lines accordingly relative to the datum point.

The paper professionally used for mapping is usually linen backed and is stretched and glued to the table top, drawing the edges of the paper underneath. This setting up takes substantial preparation time.

It is an object of the present invention to provide an educational apparatus or toy plane table which is simple and robust in construction and which requires minimal skill or preparation time to set up.

According to the invention there is provided an educational apparatus or toy plane table comprising a tripod having telescopic height-adjustable legs, a table top mounted on said tripod, said table top having a plurality of paper retainers, each adapted to retain a corner of a sheet of paper, each paper retainer comprising a triangular moulding held captive in an associated aperture in the table so as usually to lie flush with or below the table surface but having spring mounting means enabling it to be selectively raised to receive a corner of a sheet of paper, and allowed to spring back to its usual position to hold said corner of the sheet of paper tightly to the table.

The table may have an aperture adjacent each corner. Each aperture may have a chamfered edge inclined downwardly towards the corner of the table, so as to enable the paper to be smoothly drawn down by the clip.

Preferably, the paper retainer is made as a one piece moulding in a resilient synthetic plastics material, the spring means being formed integrally with the paper retainer. For example the spring means may comprise a pair of wings adapted to bear resiliently on the underside of the table top adjacent the border of the associated aperture therein.

The tripod and/or the table top may be made as mouldings in synthetic plastics material. There may be a spacer frame adapted to receive the tripod legs as a sliding fit, to retain them stably in position and prevent excessive flexing.

A central mount may be provided in the tripod or on the underside of the table top to mount a plumb bob. The tripod legs may be centrally slotted, whereby the plumb bob may be used to check their vertical alignment.

A recess may be provided in the table top and a compass may be fixedly mounted in the recess.

An alidade may be provided for use with the plane table and may comprise a one piece synthetic plastics moulding having a body portion and a pair of integrally formed uprights, each having a sighting slot, whereby the body portion can be aligned with an object viewed through both slots, the body portion providing a drawing guide (for example a slot) to mark the line of sighting on the paper.

The alidade may also have a distance scale thereon.

One sighting slot may have an upright sighting blade or wire.

A plane table embodying the invention will now be described by way of example only with reference to the accompanying drawings in which:- FIGURE 1 is an exploded perspective view of a plane table and alidade; FIGURE 2 is an enlarged scrap perspective view of a corner recess of the plane table top; FIGURE 3 is an enlarged perspective view of a corner clip engageable with the plane table corner recess.

Referring to the drawings, a plane table generally indicated at 10 comprises a tripod assembly having three legs 11, each of which has an upper part 12 with an angled mounting portion 13, and a lower part 14 telescopically slidable into the upper part 12. The upper and lower parts 12,14 of the legs have central slots, aligned when the parts are slidably engaged and adapted to receive a clamping assembly comprising a wing nut 15, washer 16 and bolt 17, which can be tightened to retain the legs in a chosen relative position.

The tripod also includes a mounting plate 18 having channel shaped leg retainers 19 disposed radially on its underside to slidably receive and retain the angled mounting portions 13 of the respective legs 11.

The tripod may optionally be completed by a spacer frame 20 through which the lower parts of the legs 14 are slid and which locates against the bottom ends of the upper parts of the legs, to provide stability to the tripod. It will be appreciated that the plane table is made as moulding in synthetic plastics material and the legs 11 are therefore not particularly stiff or rigid.

A table top 21 is fixedly mounted on the mounting plate 18 of the tripod, so as to be centred thereon. The table top 21 is rectangular in shape and a triangular aperture 22 is provided adjacent each corner, as shown in more detail in Figure 2. The aperture 22 has a location tongue 23, adapted to cooperate with a corner clip 24 to retain it in position. The corner clip 24 is shown in Figure 3 and is triangular in shape, so as to be an easy sliding fit in the aperture 22. The corner clip 24 has a part-circular flange 25 preventing it from passing through the aperture 22 when it is inserted upwardly, and the location tongue prevents the corner clip 24 from being released once it has been attached by pushing from below the table. The corner clip 24 is made of resilient plastics material, A pair of wings 26 bear on the underside of the table and act as integral springs to bias the corner clip 24 downwardly so that it usually lies just below the surface of the plane table top 21.

A sheet of paper can be clamped easily onto the plane table top 21 by manually pushing each of the corner clips upwardly against the biassing force of the spring wings 26, so that the corner clip 24 stands proud of the table top 21 and then inserting the paper in the clip. When the clip 24 is released it withdraws below the table top level, drawing the paper taut. A bevelled edge 27 on the aperture 22 assists in guiding the paper.

The table top is provided with a recess 28 at one side to receive a compass, which is fixed in the recess.

A plug mount 29 push-fitted into the centre of the mounting plate 18 holds a line on which a plumb bob 30 hangs for the purpose of ensuring verticality of the tripod in use and to centre the table over a datum point on the ground. The line of the plumb bob can be viewed against the central slots of each of the legs in turn so as to ensure that they are in vertical planes considered radially of the tripod.

An alidade 31 is provided for drawing on the paper (not shown) mounted on the table top 21.

The alidade 31 comprises a one piece synthetic plastics moulding having a body portion 32 and a pair of integrally formed uprights 33,34, each having a sighting slot 35, whereby the body portion 32 can be aligned with an object viewed through both slots 35, the body portion providing a drawing guide 36 (in the example shown, a slot) to mark the line of sighting on the paper. The alidade may also have a distance scale 37. The sighting slot of one upright 33 may have an upright sighting blade or wire 38.

The apparatus is light in weight and can readily be dismantled into compactly shaped components. Assembly can also be carried out easily, and minimal time is needed to set up the apparatus for use, since the paper can be fixed quickly and retained taut by the use of the clips described, the plumb bob can quickly be push-fitted and the compass is permanently fixed in place ready for use. The components, including the alidade, are all robust and durable in use.