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.

 

Overlay Topology templates alter objects in the drawings upon which they work, in contrast to Overlay templates which alter fields in the drawings upon which they work.

 

Our task in this example is to create a map that shows cities in the "lower 48" states of the US, a region often referred to by the abbreviation CONUS, for "Continental United States."   The region is a convenient one for examples because it fits on rectangular computer monitors without excessive white space to cover the great geographic extent of various US colonial holdings, territories, not-quite-states and so on.

 

We already have a drawing that shows the lower 48 US states as areas.   We have imported a drawing of US cities published as a shapefile by the National Atlas of the United States, a US government web site.   Our task is to trim that drawing to remove all cities that fall outside of the lower 48 states.

eg_topology_overlay_intersect01_01.png

 

Our map uses a Bing streets imageserver layer as a background.   City points have been formatted with a different color for each state.

 

Looking at the two drawings together as layers in a map we see that there are many city points outside of the lower 48 states, spanning the entire world, from Alaska to distant settlements on Aleutian islands on the other side of the +/- 180 meridian that conventionally marks the end of West and the beginning of East.   There are also clumps of dots from US colonial holdings in the Caribbean in the Virgin Islands and Puerto Rico, dots for Hawaii and, of course, many dots for cities in Alaska.

 

eg_topology_overlay_intersect01_02.png

 

Zooming in we see how most dots fall within the lower 48 states.

 

eg_topology_overlay_intersect01_03.png

We would like to trim the cities drawing so that only dots that fall within the lower 48 states are left, as seen above.   But even that is not quite enough.

 

If we zoom in some more we can see that the data sets do not quite match.

 

eg_topology_overlay_intersect01_04.png

 

In particular, the US states drawing is a simplified representation of US states that shows only the main areas of states.  It does not include islands and other small areas on which may be located towns.   

 

eg_topology_overlay_intersect01_04a.png

 

As seen above with the Bing layer turned off for greater clarity, some dots representing towns and cities do not fall within state areas, for example, those on Nantucket Island and on the island of Martha's Vineyard.  We will delete those as well.

 

eg_topology_overlay_intersect01_05.png

 

Opening the cities table, we hide most of the fields to provide a more legible display.   Each city has a name and a two letter state code.

 

eg_topology_overlay_intersect01_06.png

Opening the states table we see that it includes a field with the name of the state.   We will transfer that field as part of the Intersect operation.

 

ico_nb_arrow_blue.png  Click the cities drawing tab in the map to make it the active drawing.

 

In the Contents pane we choose the Transform panel.     It opens automatically pre-loaded with the Geom field for the cities drawing as the Target field.   We enter the letters over into the filter box, to reduce the very long list of templates to those which contain that text, such as the Overlay Topology, Intersect template we will use.

 

eg_topology_overlay_intersect01_07.png

 

We choose Overlay Topology, Intersect as the template.   The Overlay box automatically loads with US states since that is the only other drawing in the map.

 

eg_topology_overlay_intersect01_07a.png

 

As soon as we choose the template the panel previews in the drawing using blue preview color the objects that are the target, that is, the cities points.  That provides a visual check against errors.

 

eg_topology_overlay_intersect01_07b.png

 

We would like to specify which fields to transfer as part of the intersect, so we will click on the Options button to launch the Transform Options dialog.

 

eg_topology_overlay_intersect01_08.png

 

We first will shorten the names to be used for the new components to be created by the template as the default suggestions are too long for our taste.

 

eg_topology_overlay_intersect01_09.png

 

We do not need the mfd_id or Geom fields from the overlay drawing so we will Ctrl-click on each of those fields in turn to select them.   We then double-click into either of the transfer method cells for the selected records and then in the menu of methods that pops open we choose ignore.      Changing a transfer method for any selected row changes it for all selected rows.  

 

Depending on the data type of the selected field we double-clicked, the menu of transfer options can be very long or relatively short.

 

eg_topology_overlay_intersect01_10.png

 

We are not interested in copying areas from the US states drawing; we just want to use those areas to clip the points in the cities drawing.    Changing the transfer method to ignore means those fields will not be transferred into the components created by the template.  To keep the display from being cluttered an ignore choice is shown as a blank in the transfer column for fields that are to be ignored.   

 

We must leave at least one field to be transferred from the overlay drawing for the template to work.   In this case we will choose the STATE field to be copied over so that will add the name of the state in which it is located to each city point.   If later on we decide we do not need that field in the new table it is easy to delete from the table's schema.

 

eg_topology_overlay_intersect01_11.png

 

We double-check our work and then press OK to confirm the changes we made to the options.

 

eg_topology_overlay_intersect01_12.png

 

Back in the Transform panel we press Add Component to create a new component that is the result of the template.

 

eg_topology_overlay_intersect01_13.png

 

Two new components, a table and a drawing to show that table's objects, are created in the Project pane using the names we specified in the Options dialog.

 

eg_topology_overlay_intersect01_14.png

 

If we double-click the cities tab to turn off display of that layer and then we drag and drop the new cities Intersect drawing into the map we can see that the offshore points which do not fall within any state area have been deleted.    We have styled the new cities Intersect layer to use a bright green fill color for points.

 

eg_topology_overlay_intersect01_15.png

 

If we right-click the cities Intersect layer and choose Zoom we zoom to fit that layer.   We can see that all points outside the lower 48 US states have been deleted.   

 

eg_topology_overlay_intersect01_15a.png

 

If we like, we can double-check by zooming out to see the entire East / West extent of the Bing layer, confirming there are no more city points outside the "lower 48" states.  Our task has been accomplished.

 

eg_topology_overlay_intersect01_16.png

 

As part of the intersect operation we copied over a STATE field from the US states drawing to the newly created table.  Opening the new cities Intersect Table table, and then hiding all the fields in which we are not interested,  we can see that the STATE field has been brought over automatically from the US states drawing that we used as the overlay drawing.  In the new table it has been renamed o_STATE to indicate it is the result of the overlay topology operation.   

 

The values in the o_STATE field have been automatically matched so that city points which fall within the area of a given state receive the value of that state's name in their o_STATE field.  In that respect, an Overlay Topology, Intersect operation has the same effect on transferred fields as does the "fields only" Overlay Intersecting operation.

 

Notes

CONUS means Continental - The region shown in our map of the "lower" 48 US states is referred to by the US government (including the US military) using the acronym CONUS.  By US Federal law the acronym CONUS means the "Continental United States" and refers to the lower 48 US states plus the District of Columbia but excluding Hawaii, excluding various overseas colonies of the US and excluding Alaska.      

 

Some people, perhaps embarrassed at the implied geographic illiteracy of implying that the mighty Alaskan peninsula is not an integral part of the continent of North America, will say the acronym means "Contiguous United States."  Endless web sites will provide false information such as ""Many people assume that acronym stands for CONtinental U.S. when it really stands for CONtiguous U.S."   Not so.   

 

Visiting the Code of Federal Regulations (CFR) at 45 CFR chapter 300, Part 300-3 "Glossary of Terms," section 300-3.1  we read the definition "Continental United States (CONUS) - the 48 contiguous States and the District of Columbia."  By law, as far as the US Code of Federal Regulations defines acronyms, Alaska is not a part of the continental United States.   Various style guides issued by the US military and US government bureaucracies also confirm that the acronym CONUS does indeed means Continental United States.  

 

What is that empty vertical lane in the data?  - Looking at the new cities Intersect drawing we see what appears to be a vertical lane from Missouri up to the Northern border where few cities are located.  Why would there be such an empty, vertically oriented lane in the US?

 

eg_topology_overlay_intersect01_15.png

Expanding the window to get a better look at the data we can see that the apparently empty vertical lane is just a consequence of how points are averaged out for display when too many of them are in the data to fit into all the pixels that are available in a given region of the monitor.

 

The illustration below shows a more zoomed-in view with city points reduced in size, which makes it clear there is no such empty vertical strip in the data.   But we can see what appear to be horizontal lines of towns and cities running across the Great Plains, for example, from Nebraska (light beige) to Wyoming (light gray).

 

eg_topology_overlay_intersect01_17.png

 

Those horizontal lines of towns and cities are real data.  If we zoom in and increase the size of the city points we can see them even better.   

 

eg_topology_overlay_intersect01_18.png

 

The horizontal lines of towns are the result of settlement patterns as the European population of the United States expanded Westward following the lines of railroads.   

 

Settlements naturally were founded and survived along the East to West railroad routes.   Many of those original railroads are long gone but their routes remain visible in the lines of towns and cities on the Great Plains.   Some additional lines can also be made out along the routes of railroads through the mountain passes and deserts of the "basin and range" geography further West.

 

See Also

Style

 

Contents Pane

 

Contents - Transform

 

Transform Options

 

Transform Templates

 

Transform Templates - Drawings

 

Transform Templates - Geom

 

Transform: Overlay

 

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: Union Areas - Combine multiple area objects into a single area.   A drawing of French regions shows some regions as more than one area.  We would like each region to be one area so the table of regions has one record per region.

 

Example: Use a Transform Expression to Create Buffers in a Drawing - Use the Expression tab of the  Transform panel to create three different sizes of buffers for different lines in a drawing and then automatically create a query which does the same thing.  Includes examples of using the Add Component button and also the Edit Query button.