COD Tutorial: How to create polylines based on administrative levels from COD-AB polygons
- 1 Introduction
- 2 Methodology
- 2.1 STEP 1: Create the polyline layer.
- 2.2 STEP 2: PREPARE THE P-CODE FIELDS IN THE POLYLINE SHAPEFILE
- 2.3 STEP 3: POPULATE THE POLYLINE P-CODE FIELDS
- 2.4 STEP 4: CALCULATE THE ADMINISTRATION LEVELS
- 2.5 ADMINISTRATIVE LEVEL 0
- 2.5.1 SHORELINE TREATMENT
- 2.6 HIGHER ADMINISTRATIVE LEVELS
- 2.6.1 Administrative level 1
- 3 Related articles
Introduction
One of the desirable features for COD-AB (and COD-EM) datasets is a line layer with an 'admLevel' attribute field designating the administrative level of each line segment.
Administrative levels |
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Administrative boundary Common Operational Datasets (COD-ABs) and Edge-matched Common Operational Datasets (COD-EMs) model the hierarchical administrative structure of a location with nested (often multi-part) polygon layers. The 'highest' administrative level, ADM0, generally corresponds to the entire country or location, for which there is one (normally multi-part) polygon feature. The next administrative level, ADM1, represents the first sub-national administrative level. Terminology for the ADM1 feature types varies widely and could be 'state', 'province', 'oblast', 'governorate' or another term. COD-AB and COD-EM datasets may have as many as five administrative levels (ADM0 to ADM4) depending on relevance an data availability. |
The COD-AB and COD-EM lines layers include a single lines layer, typically composed of the lines forming the lowest administrative level polygons.
The 'admLevel' attribute represents the administrative level or depth of each line feature. Features with 'admLevel' 0 represent international borders (but '99' is specially substituted to represent coastlines); 'admLevel' 1 represents lines between distinct administrative level 1 units, etc.; and integers between 10 and 90 are reserved for special-status boundaries. These files are necessary for the best cartographic symbology. The alternative is to symbolize and sequence all polygon boundaries so that higher level boundaries have dominant symbols, revealing lower level boundaries in between. (Even if these are carefully disguised by symbol size and order, shorelines cannot be symbolized properly.) ITOS makes these line shapefiles when they process COD-AB and COD-EM datasets but if a country is not yet and 'enhanced' COD the coded line file can also be made with the following instructions.
The process may be unnecessary for island or landlocked countries containing only administrative level 0 polygons.
Polygon symbolization example
Colombia ("Colombia - Subnational Administrative Boundaries" COD-AB available on HDX here)
The Colombia COD-AB features administrative levels 0, 1, and 2. To demonstrate the weakness of cartogprahy that does not use the 'admLevel' lines layer, these levels will be symbolized with successively wider and darker lines, with the administrative level 0 (international / shoreline) boundary widest and boldest.
Colombia administrative level 0 (international / shoreline) polygon symbolization.
Colombia administrative level 1 (department / capital district) polygon symbolization.
Colombia administrative level 2 (municipality) polygon symbolization.
Colombia, crude administrative level 0, 1, and 2 polygon symbolization. The shoreline is not clearly defined.
Preferred lines symbology example
Colombia, administrative level 0, 1, 2, and shoreline lines symbolization
The preferred symbolization applies distinct symbologies to specifically labelled line features. If a line separates polygons with different administrative level 3 features, but the polygons on each side have the same administrative level 2 P-codes, then that line is an administrative level 3 boundary.
For example, if a feature in an Australian lines shapefile separates different counties (administrative level 2 features), but the polygons on each side have the same State (administrative level 1 feature) P-codes, then that line is an administrative level 2 boundary.
No line feature relies on its symbol obscuring other layers, and special features like shorelines or disputed boundaries can be symbolized distinctly at the same level.
Methodology
It may be useful to consider a narrative description of the methodolgy
The polyline layer is created from the lowest administrative level polygon layer. This layer type contains individual lines between pairs of adjacent polygons and along the outer edges of peripheral polygons.
An attribute field in the lines table, 'admLevel' is populated with the administrative level of each line feature.
STEP 1: Create the polyline layer.
The goal is to create a lines shapefile with individual line features between nodes and left and right polygon identifiers. (Polygon shapefile exterior features have their left polygon identified as '-1'.) This can be performed with ArcGIS 'Advanced' license or the registered version of ET GeoWizards. This process is not possible with ArcGIS desktop or ArcGIS Pro 'basic' or 'standard' licenses.
The input features will be the polygon shapefile with the lowest available administrative level. This is the only shapefile that can provide all desired arcs.
Run the ArcGIS ‘Polygon to Polyline’ tool (Toolbox > Data Management Tools > Features > Polygon To Line) with: "Input Features" = (lowest administrative level polygon shapefile); "Output Feature Class" = (as desired); "Identify and store polygon neighbouring information" selected; and all environment settings set as default.
(Illustrations are for ArcGIS Pro.)
Sample input parameters:
Sample output (partial):