Transform: Overlay

Overlay templates become available in the Transform panel in the Contents pane when the focus is on a drawing layer in a map.  That active drawing is called the target drawing.  Based on geometric relationships with objects in a second drawing in the map, called an overlay drawing, the templates transfer field values from the overlay drawing into the target drawing.

 

il_overlay_process.png

 

 

For example, the Country field value of France in an overlay drawing might be transferred to all objects in the target drawing that intersect France.   Those objects in the modified target drawing will have a new Country field added to their tables that shows they are part of France.

 

The GIS jargon word overlay can obscure what is going on.   Replace that word with the phrase Transfer field values to and what is going on becomes clearer.

 

Overlay Adjacent

Transfer field values from adjacent objects in the overlay to objects in the target.

il_overlay_adjacent_fr.png

Above: Take fields from the adjacent area in the blue overlay drawing and transfer them into areas in the yellow target drawing.

 

Real life example:  A country requires special permits for any construction on real estate parcels that are next to (that is, adjacent) to a wetlands area.   The overlay is a drawing of wetlands areas with a Wetlands field that has a value of Yes for each wetlands area.   The target is a drawing of real estate parcels.   

 

The Overlay Adjacent template creates a modified target drawing where each parcel adjacent to a wetlands area also has a new Wetlands field with a value of Yes, in addition to all the other fields that parcel had in the original target.

 

When the government receives a request for a construction permit for a particular parcel it can quickly check to see if the Wetlands field for that parcel contains Yes, and if so apply the special permit process.

Overlay Contained

Transfer field values from contained objects in the overlay to objects in the target.  

il_overlay_contained_fr.png

Above: Take fields from contained points in the blue overlay drawing, sum the field values and transfer the result into the area in the yellow target drawing.

 

Real life example:  A target drawing shows provinces in France as areas with information in fields for each province such as the name of the province.  However, the drawing does not have a Population field giving the total population of the province.  The overlay drawing contains points for each populated place in France and includes information fields for each point giving its name, Population and other information.

 

The Overlay Contained template creates a modified target drawing with a new Population field with the sum of the Population values for populated place points contained by each area.  We can specify that the sum of the values is to be used in the Options for the template.

 

See the Example: Overlay Contained topic.

Overlay Containing

Transfer field values from containing objects in the overlay to objects in the target.

il_overlay_containing_fr.png

Above: Take fields from the containing area in the blue overlay drawing and transfer them into the points in the yellow target drawing.

 

Real life example:  A target drawing shows points in the US where each point has a street address.   The overlay drawing shows Census Block Group area with each area having a Block Group number.   We want to add the correct Census Block Group number to each point in our target drawing.

 

The Overlay Containing template creates a modified target drawing where each point within a particular Census Block Group automatically receives the containing area's Census Block Group number.

 

See the Example: Overlay Containing topic.

Overlay Intersecting

Transfer field values from intersecting objects in the overlay to objects in the target.

il_overlay_intersecting_fr.png

Above: Take fields from the intersecting area in the blue overlay drawing and transfer them into the lines in the yellow target drawing.  Ignore adjacent areas and non-intersecting lines in the target.

 

Real life example:  We will take a truck on a transcontinental journey and we would like to minimize the amount of total road tax we will pay on the journey.  We have several possible routes from which to choose.

 

A target drawing shows lines that represent different routes through the road network from our start location to the destination   The overlay drawing shows states as area objects.  Each state has a Road Tax which is charged to any truck passing on a road through that state.   We would like to add a field to each route line that gives the sum of all of the Road Tax values for every state through which the route line passes.  That will tell us the total tax that will have to be paid for each route.

 

The Overlay Intersecting template creates a modified target drawing where each route line acquires a new Road Tax field with the sum of the Road Tax values for each of the intersecting state areas through which the route line passes.  We can specify that the sum of the values is to be used in the Options for the template.

Overlay Touching

Transfer field values from touching  objects in the overlay to objects in the target.

il_overlay_touching_fr.png

Above: Take fields from the touching area in the blue overlay drawing and transfer them into the points and areas in the yellow target drawing.  Anything that is containing or adjacent is also touching.

 

Real life example:  We are studying the proposed path of a crude oil pipeline and we would like to know what water features may be affected.   A target drawing shows wells, rivers and lakes as points, lines and area objects.   The overlay drawing shows the proposed route of the pipeline as a line object, with a field called Pipeline giving the name of the proposed route.    We would like to know each water feature that the route of the pipeline touches.

 

The Overlay Touching template creates a modified target drawing where each well, river or lake which the pipeline touches acquires a new Pipeline field that includes the name of the proposed pipeline route.   The default Options setting of copy will transfer the name of the proposed pipeline route.

 

Rules:

 

 

 

Overlay templates are different from Overlay Topology templates:   

 

 

Also:

 

 

Overlays in Action

A classic example of using overlays is to transfer the value of a field in an area to all the points that fall within the area.

 

il_spatial_overlay01_05.png

 

Suppose we have a map of France with two drawings as layers, one drawing that shows Cities as points and the other that shows Provinces (regions) as areas.

 

il_spatial_overlay01_04.pngil_spatial_overlay01_02.png

 

The table for Cities gives the name of each city and the table for Provinces gives the name of each region.    We would like to create a new, improved table for Cities that has both the name of the city and the name of the region in which that city is located.

 

il_spatial_overlay01_06.png

That is an easy task for which we use the Overlay Contained template.  See the Example: Overlay Contained topic for a step by step example of how that is done with just a few mouse clicks.

Adjacent, Contained, Intersecting and Touching

It is easy to take for granted the meaning of such common words but all the same there are a few technical nuances as follows:

 

Overlay Adjacent

Adjacent means that both objects have at least one boundary location in common but have no interior locations in common.  

 

An object that is adjacent to another object is touching, but it does not intersect the other object.  Objects that are contained by or are containing another object are not adjacent.

Overlay Contained

Contained means that all locations of the contained object are entirely within the containing object.

 

An object that is contained by another object is touching and intersecting but it is not adjacent.

Overlay Containing

Containing means that all locations of the contained object are entirely within the containing object.

 

An object that is containing another object is both touching and intersecting that other object but it is not adjacent to that other object.

Overlay Intersecting

Intersection means that both objects have at least one interior location in common.  

 

An object that intersects another object is touching, but it is not adjacent to the other object.  Objects that are contained by or are containing another object are also intersecting.

Overlay Touching

Touching means that both objects have any location in common.  

 

One object that is touching another object is intersecting, but it may or may not be adjacent to the other object.  Objects that are contained by or are containing another object are also touching.

 

Consider the following example diagram, where the rectangular yellow area is in the target drawing and the other objects are in green, purple and blue overlay drawings.  The small diamond, square and circle shapes mark the location of point objects.   The line objects and purple square are placed directly upon or incident to the boundary of the yellow area..

 

il_adjacent_intersect_contained.png

The geometric relationships seen above:

 

 

Area objects are adjacent when only locations on their boundaries are coincident but no interior locations are in common.   The purple square is thus adjacent because only locations on the purple square boundary are coincident with locations on the yellow area boundary.    The green square in the corner of the yellow area is not adjacent because parts of the green square are coincident with the interior of the yellow area.

 

Line objects are adjacent if one of their end coordinates is coincident with the boundary of the yellow area.   The purple line is adjacent to the yellow area because only the end point of that line is coincident with the boundary.  None of the other lines are adjacent to the yellow area, because some part other than just the ends are coincident with some part of the yellow area.   

 

Points that are located on the boundary are adjacent to the yellow area, so the purple point is adjacent.

 

The green square in the corner is contained by the yellow area even though part of the coincident locations are on the boundary.   Both green squares are contained by the yellow area since there is no requirement that only locations in the interior count and not coincident locations on the boundaries of the objects.  Likewise, the green L-shaped line is contained by the yellow square even though it is located entirely on the boundary of the yellow area.

 

Objects that have any part in common with the interior of the yellow area are intersecting the yellow area.   All of the green objects except for the green L-shaped line are intersecting the yellow area.  The green L-shaped line does not intersect the yellow area because no part of the green L-shaped line extends into the interior of the yellow area.   All of it is on the boundary only.   All of the blue objects clearly cover part of the interior of the yellow area, so all of the blue objects are intersecting the yellow area.

 

The black line clearly is obviously not completely contained within the yellow area, and because more than just the very end of it is coincident with the yellow area boundary it is not adjacent, either.   It does not intersect the yellow area because no part of the black line is coincident with an interior location of the yellow area.   But the black line is touching the yellow area.

 

 

Notes

Multiple records - Illustrations of tables might show more than one record for various Provinces.   Why is that?  GIS data will often use multiple records for each area that makes up a particular region.   For example, the region of Bretagne (known in England as Brittany) includes many islands, each of which is a separate area object in this data.

 

Old Data - The illustrations in this topic use data from the US military, which show the regions of France as they were before 1 January 2016, when a law passed in 2014 took effect that reduced the number of regions in France from 22 to 13.  

 

Provinces vs. Regions -  The drawing is called Provinces and not Regions because it was clipped out of a larger data set showing provincial boundaries for the entire world.  Around the world sub-national divisions such as US states or French regions are called many different things, but the word provinces seems to have become a reasonably generic word that militaries and others engaged in mapping often use.   State is rarely used because in many cultures it is a synonym for country and thus does not capture the notion of the administrative sub-units of a given country.

 

See Also

Contents Pane

 

Transform

 

Contents - Transform

 

Transform Options

 

Transform Templates

 

Transform Templates - Drawings

 

Transform Templates - Geom

 

Transform: Overlay Topology

 

Example: Overlay Contained -  A frequent use of overlays is to sum the values of many points that fall within an area and to transfer that sum to a new field for an area.  In this example we take a drawing that has cities in the US with a population value for each city.  We use Overlay Contained  to sum the population of each city within a state and to transfer that sum to a total population for the state.

 

Example: Overlay Containing - One of the most common uses of overlays is to transfer fields from areas to points that are contained in those areas.    Tasks such as transferring a census block group number or zip code number from a drawing of areas to points that fall within each area are extremely common.   In this example we transfer the name of a French region  to the points that represent cities which fall within each region.

 

Example: Overlay Topology Intersect - In this example we use the Overlay Topology, Intersect template in the Transform panel to trim a drawing of points so that all points which do not fall within areas in a second drawing are deleted.   The drawing of points we trim will become the US cities drawing that is used in the Example: Overlay Contained topic.