Convert SID to GBR

Technical note

How SID and GBR differ in practice

Geospatial coordinates are absolute positions on the earth, and they are large: a projected easting runs to six or seven digits, while latitude and longitude are small decimals. GBR expects a drawing laid out near its own origin in millimetres or inches. The geometry itself converts correctly, but it can land tens of kilometres from 0,0, or arrive at a scale where an entire county is forty units across. Zoom to extents first, then measure one known distance before dimensioning anything. A line in a SID image is several pixels wide, so tracing it can mean different things and the mode is yours to choose. Centreline returns a single line running down the middle of each stroke, which is what a scanned drawing needs: one entity per line, snappable and dimensionable, rather than a long thin rectangle around it. Outline follows both edges and returns closed shapes, which suits clipart, logos and anything that will be filled or cut. A precision setting controls how closely the traced paths follow the pixel edges. One thing to plan for: vectorised output from this path is black and white, so colour in the SID does not survive as colour in the GBR.