Offsets and ties are simple survey techniques that require two divers.  The first step for both methods is to set out a straight line through the centre of the area to be recorded, this is called a baseline. Distances are measured from this baseline to features on the site using a tape measure or measuring rod. To position using an offset a single measurement is made from the feature to the baseline, meeting where the offset measurement is at right angles to the baseline. To position using ties, two or more measurements are made from the feature to two or more separate points on the baseline.

Offsets and ties can be used for assessment surveys over large areas or for picking up detail in recording surveys. The position accuracy of offsets is limited, it is difficult to estimate the right angle to be measured at the baseline and setting it out correctly would be very time consuming. If improved accuracy is required then an alternative technique should be used.

Because of the problems in measuring a right angle for an offset, ties should be used where the feature is further than 3m from the baseline and an accurate position is required. A problem that affects both methods is that long baselines can move sideways and do not provide a fixed reference for the offset or tie measurements.

Additional tie measurements made to each feature can be used as checks on other ties or offsets. If more than one point on an object is positioned then it can be oriented on the plan, if the length of the object is also measured then this can be used as a check measurement.

The offsets and ties techniques are recommended for use in confined areas such as in rock gullies or within ship structure.

Drawing up a plan view based on the offset and tie measurements requires a scale rule, pair of compasses and a set square.

If drawing on paper:

1. Select a suitable scale for your drawing.
2. Draw the baseline to scale on the plan, label the points at each end of the baseline.
3. For each feature, mark off the distance measurements along the baseline to scale.
4. For an offset measurement, draw a line at right angles to the baseline on the correct side of the baseline using the set square. Set the compasses to the offset measurement using the scale rule and strike off the distance along your offset line, this is the position for your feature.
5. For a tie, set the compasses to the first measurement using the scale rule then strike an arc on the paper on the correct side of the line. Set the compasses to the second measurement and strike a second arc through the first. The feature can be plotted where the arcs cross.
6. Add the names for the features, a scale and a north arrow to the drawing.

If drawing up on a computer:

1. If a typical drawing program is being used then the scale does not need to be specified.
2. Draw the first control point at a convenient co-ordinate such as 500,500 to ensure all features have positive co-ordinates.
3. The second control point can be drawn relative to the first, the correct distance away. If the orientation is not known then draw the baseline vertically or horizontally.

1. For an offset measurement, draw a line at right angles to the baseline on the correct side of the baseline.
2. For ties, taking the firs tie measurement draw a circle centred around the point on the baseline with a radius equal to the length of the tie. Repeat this for the second tie measurement. The feature can be plotted where the circles cross.

1. Add the names for the features, a scale and a north arrow to the drawing.

If the bearing of the baseline is known then the site can be drawn in its true orientation. Some drawing programs cannot draw lines at right angles to other lines. In this case draw the site with the baseline horizontal or vertical then rotate the whole survey to the correct bearing once drawn.