Map overlays & Geoprocessing Learn about spatial analysis functions

Map overlays & Geoprocessing Learn about spatial analysis functions overlays, clipping & buffering Use overlays to analyze multiple spatial criteria Understand differences between spatial joins and overlays Geoprocessing with menus, Arctoolbox and model builder

Map Overlays Combines layers into a single output 2 categories Outputs that combine attributes includes INTERSECT and UNION Outputs that don’t combine attributes includes CLIP and ERASE collectively known as extraction functions

Overlay Functions: Overlays of two feature classes creates a new feature class with attributes from the two originals...THIS IS WHAT GIS IS ALL ABOUT!

Extraction functions: Clip- THINK COOKIE CUTTER with the inside of the cutter being what is preserved and everything outside is clipped away, leaving only the attributes inside Erase- THINK DONUTS with the part that we are interested in being the donut, and the hole being erased from the set of attributes that we are dealing with

Clip example Polygon shapefile used to “clip” line shapefiles; i.e.,create a subset of the of the lines that lie within the confines of the polygon

IMPORTANT! Attributes within the overlay or extraction functions are updated automatically USER defined attributes, such as the length of the road passing through a state park will not be accurate upon clipping or erasing, and have to be redefined using the calculate geometries function

Overlays with attributes take features and can split them into smaller features that look like the original feature. The blue line here is split into three line segments, each within the specific land use polygons.

Each line is now given attribute data in the attribute table to correspond to the polygon in which it now resides

Unions example: Creates polygons for every possible combination of features from the two layers while creating a new layer with uniquely shaped polygons

Intersect: creates new polygons only where the three source polygons overlap, excluding everything else result Source polygons

Intersecting various geometries of features can yield a wide array of results a) Polygons yield points, lines or other polygons; b) Lines can yield points or lines; c) Polygons and lines can yield points or lines

ARRRRGH! SLIVERS! Sometimes when layers represent similar shaped boundaries, they don’t match perfectly due to scales variations between the two data sets (e.g., counties map vs state map outer edges. This is an artifact and not real so those slivers can be corrected by using a tolerance setting. Careful because tolerance combines all values that are close, not just the ones you want Sometimes slivers are real... It is difficult to determine at times if they are real or not

Dissolve can be useful in this instance by getting rid of extraneous data points that share a common attribute- street name in this instance. Again- caution on using this because they sometimes are located there for reasons that may not be apparent at first glance

Dissolve takes polygons that are adjacent or overlap (BEFORE) and uses attributes within each that are identical to create larger polygons (AFTER)

Buffers A) around points (feature classes); and B) around lines (also feature classes).

Buffers around roads creates a mess... Until you dissolve the boundaries between the various buffer zones created

Many of these processes are computationally extensive i.e., your PC will cringe at the thought of doing the tasks you will be asking it to do Do any queries and or selections before doing any of the overlays to minimize the computational tasks especially buffers and dissolves.

Append- tables must match precisely before applying A tool used to combine features of two or more layers into a single new feature of the same class Map of Polygons 1 + map of Polygons 2 = new map layer of combined Polygons Merge is very similar, except that it allows the user to specifically determine what attributes fields to include rather than insisting that the table match exactly. Its more flexible than the append tool

Map overlays & Geoprocessing Learn about spatial analysis functions