Snapping option issues
Since my department now uses all 3 GIS softwares, I decided to compare notes between ArcGIS, Mapinfo and QGIS in anticipation of queries. Today, I looked at snapping tolerance and found out that both ArcGIS and MapInfo snapping unit are only available in pixels whereas in QGIS, user can opt between pixels or mapunit have. (I am informed by blog member Saya Suka GIS below that this is not quite true for ArcGIS) Then again, the QGIS User Guide states that "the advantage of choosing pixels is that the snapping tolerance doesn't have to be changed after zoom operations". Next, I found out that the default snapping tolerance of ArcGIS and MapInfo is respectively 3 and 5 pixels whereas for QGIS, it is not set at 0.000000 either in pixel or mapunit. This means that the moment a ArcGIS and MapInfo layer is created, snapping of a digital feature is already possible and is comparatively more accurate in ArcGIS than in MapInfo by a ratio of 3:5. This does not exist in QGIS and if the user wants snapping, then customize a preferred tolerance. So what is the de facto set tolerance? Well, the more zeros there are, the more accurate the layer becomes where 0.001 has a higher tolerance than 0.1 but that level depends on the user's preference, for example, I was told that the Malaysian Land and Survey Dept. (JUPEM) for cartography purpose uses a snapping tolerance of 0.000001 pixels (you have to confirm it from the horse's mouth). This means that if a value-added layer was made from JUPEM's cadaster map, any amendment concerning topology ought to follow the snapping tolerance of the custodian to ensure that accuracy is maintained. No one is said another agency cannot use a different snapping tolerance level but there are pros and cons to consider. My department has yet to establish a department snapping tolerance (policy) for all town planning agencies and I feel there should be and hope to address this issue in future. Thirdly, I also found out that a layer created with QGIS default tolerance of 0.000000 pixels when activated as a .tab file by MapInfo did not show pixel tolerance of the earlier GIS software but MapInfo's 5 pixel. This means another layer prepared at a tolerance level of say 3 pixel, if uploaded into MapInfo, unless adjusted, will stay 5 pixels. Looks like there is a need for standardization and coordination on this issue.