Author Topic: Shapefile Content  (Read 693 times)

GisDave

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Shapefile Content
« on: May 24, 2015, 12:35:31 am »
I am just becoming familiar with the GIS world so this is a really basic question.

I used EasyGIS.NET to read a US Census shapefile (which was super easy and worked perfectly!) but I saw that a single shapefile record had multiple polygons in it.  The different polygons had varying numbers of vertices so I thought maybe they were just different levels of precision describing the same general area but when I plotted the polygons on google maps I saw that this was definitely not the case.

I had assumed that a single shapefile record (as enumerated by EGIS.ShapeFileLib.ShapeFileEnumerator.MoveNext) would contain only one polygon so now I am confused as to what each polygon represents.

Thank you for any light you can shed on this for me!

wf

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Re: Shapefile Content
« Reply #1 on: May 26, 2015, 03:42:07 pm »
Each record of a Polygon shapefile can contain multiple polygons, which are known as "parts". Although less common, a PolyLine shapefile can also contain multiple parts per record. A shapefile record consists of spatial parts + attributes (stored in the .DBF file)

The reason for this is because each a record may indeed be made up of multiple parts. For example a world map may consist of a a single record for each country, but each country is made up of multiple parts/polygons. A country like the US may be represented as three main polygons - the main contiguous states, Alaska and Hawaii. Other countries like The Philippines consist of 100's of islands.