We have looked at data and the connections between the geometries (features) and their attributes in layers. One of the other key aspects in learning GIS is the analysis of relationships between layers. As we see in lab, GIS software such as QGIS and ArcPro have a multitude of tools to carry out these types of analysis.

There are many types of overlays, we will look at more of these through the semester, but we will start by looking at a few key definitions.

Buffers

Key things to remember:
– Can buffer inward or outward or both
– The resulting buffer layer can keep the attributes of the original layer (undissolved)
– Dissolving removes inner boundaries
Links:
https://docs.qgis.org/3.10/en/docs/gentle_gis_introduction/vector_spatial_analysis_buffers.html

https://pro.arcgis.com/en/pro-app/tool-reference/analysis/buffer.htm

Clip

Key things to remember:
– Emulates a cookie cutter method
– Attributes of the input layer are kept, but the cutting layers does not add attributes.
https://pro.arcgis.com/en/pro-app/tool-reference/analysis/clip.htm

Erase or Difference

Key things to remember:
– Similar to Clip, but removing from a dataset rather than cutting out
– Attributes are kept with the input layer.
https://pro.arcgis.com/en/pro-app/tool-reference/analysis/erase.htm

Intersect and Union

Key things to remember:
– All polygons and attributes are kept from both layers
– Union is all information from both layers
– Intersection is a Union mixed with a Clip (common areas)
https://pro.arcgis.com/en/pro-app/tool-reference/analysis/intersect.htm
https://pro.arcgis.com/en/pro-app/tool-reference/analysis/union.htm

Merge

Key things to remember:
– Feature types must be the same (i.e. polygon merge with polygon)
– Attributes are merged (similar to Union)
https://pro.arcgis.com/en/pro-app/tool-reference/feature-analysis/merge-layers.htm