Producing a CAD model will generally involve more than one method of data gathering. For example, total stations do not work well at very close range and they are cumbersome to use for small details. GPS is excellent for landscape survey but works less well for the details of standing architecture. Photogrammetry is difficult and expensive but excellent for details seen well in photographs. Tapes and traditional hand-measuring systems are slow but useful in areas that are inaccessible to total stations. Digitising existing plans, drawings or photographs allow existing data to be utilised. Each system has its drawbacks and each has its strengths.
When planning a data-gathering exercise a number of issues should be taken into account:
- Equipment, procedures and personnel should be kept under review to monitor the levels of precision and accuracy that are being achieved
- Where possible, modelling should proceed in tandem with survey work to ensure that data are being collected at the appropriate precision and density for the task
- An appropriate CAD layer-naming convention should be selected and followed (see Documenting the conventions)
- All factors affecting the collection of data should be fully and accurately documented (see Archiving CAD data). Potential users must be able to examine such documentation if they are to understand the quality of the data.