After data is collected, it has to undergo post-processing. A massive amount of data is collected during the typical hydrographic survey, often several soundings per square foot. Depending on the final use intended for the data (for example, navigation charts, Digital Terrain Model, volume calculation for dredging, topography, or bathymetry) this data must be thinned out. It must also be corrected for errors (i.e., bad soundings,) and for the effects of tides, waves/heave, water level and thermoclines (water temperature differences). Usually the surveyor has additional data collection equipment on site to record the data required for correcting the soundings. The final output of charts can be created with a combination of specialty charting software or a computer-aided design (CAD) package, usually Autocad.