I am wondering in which domains or countries where Differential GPS (DGPS) with post processing (not Real Time Kinematic [RTK]) is still required? It's known that RTK has been rapidly adopted by cadastral services in several countries, but I wonder if DGPS with post processing is still required? And why we still need it when we can accomplish the same task using RTK?
First, RTK stations are not always available. There are some countries where there are no or little stations, and according to this source even in the US you have states without public RTK. So you then need your own base station and if you want to make you own RTK you need an emitter in addition to the GPS receiver, so this i more expensive.
EDIT: for the centimetric level accuracy, you need a phase correction on L1/L2. Therefore you need at least one other station located at maximum 15-20 km for your receiver for the best accuracy. You can of course have your second station equiped for RTK, but in addition to being more expensive hardware, you might also loose some time because you'll need to know the exact location of your base station (if you are lucky, you know it but otherwise you need to take the average position over a long period of time). With post-processing you can compute this position at the end of your day, so you didn't loost the time to initialize.
Second, RTK signal is sometimes difficult to obtain. This is less true now, but sometimes in deep forested valleys you don't get the RTK signal even if you were supposed to. In this case post processing is necessary.
As a final remark, DGPS is used in different context, but it is primarily the fact of correcting the position captured by a GPS using information from another receiver located at a known position. DGPS can refer to any type of ground based augmentation system, including post-processing and RTK which are just two possible technical solutions to get the information for the correction. In terms of accuracy, it is the fact that you use the carrier phase information that allows you to go below the 10 cm (so DGPS is sometimes less accuracte). Not all receivers are equiped for L1/L2 carrier phase measurements, but I guess that cadastral services are.
One example is if you are setting up an RTK survey, but don't have any permanent marks.
In this situation, you set up a base station and observe for 6+ hours while you undertake your field work with a radio-connected rover (you might be able to do this disconnected?), then you can post-process to identify the coordinates of your base station. Post-processing gives you mm precision with long observation times.