You can convert an item into another coordinate system using Replicate [Edit-Clipboard]. This copies the selected items from their original datasets to the dataset in the current overlay, provided the current overlay is editable. The dataset on the current overlay should be set to the desired coordinate system.
Having said this, converting items between coordinate systems should normally be avoided, as accuracy is often lost in subtle ways. For example, when a straight LineString item is converted, both of its endpoints are converted accurately, but the LineString item is assumed to be straight in the new coordinate system. This could cause problems where the new and old coordinate systems are curved relative to each other.
Positions are transformed between coordinate systems using an intermediary coordinate system. The intermediary coordinate system is either Geocentric or Latitude/Longitude in the WGS84 datum, depending on which will give most accuracy.
When converting positions between coordinate systems based on ellipsoids (e.g. Transverse Mercator to Latitude/Longitude), the geodetic datums of the two coordinate systems are examined. If the geodetic datums are not the same then SIS will perform a geodetic datum transformation. If SIS recognises the geodetic datum, then regression polynomials are used, which are normally accurate to a few metres in the designed area. If SIS does not recognise the geodetic datum, or the position is not inside the designed area, then the seven-parameter Bursa-Wolf method is used. You can set the seven parameters to get good accuracy in your area of interest. These parameters are: X, Y and Z shift (in metres), X, Y and Z rotation (in degrees) and a correction (in ppm).
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