Interpretation of the vector data

Question

I am a new to the map processing world.

I started with QGIS and loaded a data set from Tiger products (namely US rail roads) I can view each part of the road and I can get its vector data as well. But I am unable to understand the vector data of the segment. can some one help me/ point me to any helpful resource

This is the sample set of a rail road segment:

GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]

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I want to know significance of this attributes, let say what the Degree value signifies the longitudinal distance form greenwich ? and If I want extract more properties of this line lets say its width and its latitude and longitude values how can I get from QGIS. When I did ctrl+c and ctrl+v I got following text

{wkt_geom   OBJECTID    LINEARID    FULLNAME    MTFCC
MultiLineString ((-118.48735999999996693 .....  ))  15313   1101576760604   Southern Pacific RR R1011} 

Are this vaules are its positions coordinates ?

Answer 1

This is not vector geometry data. It's a proj string that defines the projection of the geometry data.

If you want o see the geometry data in the form of a WKT string, you can select one or more features and copy (Ctrl+c) and paste (Ctrl+v) it/them into a text editor.

Answer 2

If you're new to mapping then...

As already mentioned, the GEOGCS part isn't the geometry of the objects. Instead it defines the coordinate system (or map project) being used. It applies to ALL of your data (not just a specific railroad line).

A datum is the origin or anchor point for a grid. Your datum is NAD83 (North American Datum 1983)

A spheroid is an approximation of the shape of the Earth - yes there are many possible shapes! Yours is GRS1980

A prime meridian is the line representing 0 degrees. Yours is Greenwich in London (so anything west of there is +x degrees)

The units of your data are degrees. Other units might be metres or feet. 0.017453292519943295 is the number of radians in a degree (or vice versa, I forget which).

The real question is, does this matter? The exact parameters of a coordinate system aren't usually important to the user. So what exactly do you want from the data? You say you want to extract more information - extract how? Just query it or export it to a human-readable text file? Or something else?

What is your desired result? A map? A new dataset?

And yes, when you copy/paste the geometry, I believe they are the coordinates of your data. You are looking at it in the form of Well Known Text. MultiLineString means it is a feature made up of multiple lines. So you are probably seeing (for example) a railroad from one location to another, made up of several line features.