Overlay A series of registered data layers overlaying

Overlay A series of registered data layers ‘overlaying’ each other Arguably the most important GIS analysis function

Overlay Derived from manual cartographic overlay using Mylar sheets (transparent plastic) that were physically overlaid on top of one another. -Ian Mc. Harg in landscape architecture at University of PA

Overlay • An overlay operation takes two or more data layers as input and results in an output data layer • Three types of overlay – Point in polygon – Line in polygon – Polygon on polygon • More complex in vector than in raster

Overlay • Raster overlay – layers are registered – layers have same resolution

Overlay • Point in polygon: raster – Where are there trees in the urban area? ‘Point’ ‘Polygon’ 0 0 0 0 0 1 0 0 1 0 0 - not a tree 1 - tree + Result 2 2 2 2 0 2 0 0 0 2 2 2 0 - not urban 2 - urban = 2 2 2 3 2 2 2 0 0 1 0 0 2 2 3 2 1 0 2 3 2 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree

Overlay • Point in polygon: raster – Where are there trees in the urban area? 2 2 2 3 2 0 2 0 0 1 0 0 2 2 3 2 1 0 2 3 2 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree reclassify <3= 0 3 = 1 0 0 0 0 0 1 0 0 - not an urban tree 1 - urban tree

Overlay • Line in polygon: raster – Where are there roads in the urban area? ‘Line’ ‘Polygon’ 0 1 1 1 0 0 1 0 0 - not a road 1 - road + Result 2 2 2 2 0 2 0 0 0 2 2 2 0 - not urban 2 - urban = 2 2 3 3 2 2 3 0 0 1 0 3 2 2 2 0 1 2 2 2 0 - not urban, no road 1 - not urban, road 2 - urban, no road 3 - urban, road

Overlay • Point in polygon: vector – Jacobsen plumbline algorithm Pass a line through the point and polygon if number of line/polygon intersections is: even - point is outside odd - point is inside

Overlay • Point in polygon: vector – result must include attribute information Input 1 A C Output 2 + A = B C B Point Table ID Tree A B C Elm Maple Elm Poly Table ID Cover Point Table ID Tree Cover 1 2 A B C Rural Urban Elm Maple Elm Rural Urban

Overlay • Line in polygon: vector – result must include attribute information Input Output C A + B Point Table ID Street A B C Race Arch C A = 1 2 1 D 2 B Poly Table ID Cover Point Table ID Street Cover 1 2 A B C D Rural Urban Race Arch Race Rural Urban

Overlay • Raster overlay – Boolean combination

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Agriculture Poor soil quality Agr and poor soil 0 0 0 0 1 1 0 0 1 0 0 0 0 0 1 1 1 1 1 0 0 0 1 1

Overlay • Raster overlay

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Land cover Soil quality 1 1 1 1 0 1 0 0 2 2 2 2 2 0 - residential 1 - forest 2 - agriculture + Output 0 0 1 1 0 1 1 2 2 1 0 0 0 - poor 1 - fair 2 - good agriculture + poor = 2 = forest + fair = 2 residential + good = 2

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Land cover Agriculture 1 1 1 1 0 1 0 0 2 2 2 2 0 - residential 1 - forest 2 - agriculture 0 0 0 0 2 1 1 1 1 1 reclassify 0 - not agriculture 1 - agriculture

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Soil quality Poor soil quality 0 0 0 0 1 1 2 2 1 0 0 - poor 1 - fair 2 - good 1 1 0 1 0 0 0 1 1 reclassify 0 - not poor 1 - poor

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Agriculture Output Poor soil quality 0 0 0 0 1 1 1 1 0 0 1 1 1 + 0 0 1 1 1 1 0 0 0 1 1 0 - not agriculture 1 - agriculture 0 - not poor 1 - poor 1 1 = 0 1 1 1 0 0 1 1 2 2 1 1 1 2 2 0 - not agr. , not poor 1 - agr. , not poor or not agr, poor 2 - agr, poor

Overlay • Raster overlay: polygon on polygon – where is agriculture on poor quality soil? Output Final Output 1 1 1 1 0 0 0 0 0 0 1 1 1 2 2 0 0 0 1 1 0 - not agr. , not poor 1 - agr. , not poor or not agr, poor 2 - agr, poor reclassify 0 - not agr. on poor soil 1 - agr. on poor soil

Overlay • Raster overlay: map algebra Daily high temperature Daily high humidity 0 0 0 1 1 1 2 1 0 1 1 1 2 2 2 0 - mild 1 - warm 2 - hot Hot + Humid Index 0 1 1 0 0 1 0 1 0 1 + 2 2 0 2 2 1 1 3 1 1 2 1 0 0 1 = 2 3 2 2 1 4 4 3 1 2 2 1 0 3 4 4 3 2 0 - not humid 1 - semi humid 2 - very humid 0 - very low 1 - low 2 - medium 3 - high 4 - very high

Overlay • Raster overlay: map algebra – can use addition, multiplication, etc. – can ‘weight’ certain data layers – can use any number of data layers – can’t use nominal data unless it used as inclusionary/exclusionary by reclassification! – Be careful with ordinal data - classification impacts the results of overlay! – Standardize interval/ratio data

Overlay • Vector topologic overlay: Boolean operators Intersection (and) Union (or) Identity

Overlay • Vector topologic overlay: intersection

Overlay • Vector topologic overlay: intersection (and) Agriculture Good Quality Soil Build new topology A A B B ID Owner ID Quality A Brown B Smith A good B good Area of intersection New node

Overlay • Vector topologic overlay: intersection (or) Build new topology Output A B Area of intersection New node ID Owner Quality A Brown good B Smith good

Overlay • Vector topologic overlay: union Agriculture Good Quality Soil Build new topology A A B B ID Owner ID Quality A Brown B Smith A good B good Area of union New node

Overlay • Vector topologic overlay: union Build new topology B Output C A D E ID Owner Quality Area of union New node A good B Brown good C Brown D Smith E Smith good

• Vector topologic overlay: identity Agriculture (input layer) Good Quality Soil (identity layer) Build new topology A A B B ID Owner ID Quality A Brown B Smith A good B good Area of identity New node

Overlay • Vector topologic overlay: identity Output Build new topology A B C D ID Owner Quality Area of identity New node A Brown good B Brown C Smith D Smith good