Georeferencing accuracy

[Dean Barker]

I'm trying to teach myself georeferencing but I'm not sure I'm doing it right. I kind of got the Geolocate tool and Geoimage tools working, but I'm not sure they're coming in at the right scale. I tested this theory by importing an image of the Frederick, MD airport and drawing a line along the longest runway. It comes out at around 6758'. However according to Wikipedia that runway is 5220' and that's borne out using the measuring tool on Google Maps. Why should there be such a discrepancy? Is there some setting I have wrong that would make it over 25% off?

[JAMZ21]

Hello Dean,

 

I am also learning georeferencing and I just tested this - Note my dimensions are in meters and I am using the projection EPSG 2248 for Maryland.

Also verified the runway lengths here: https://www.airnav.com/airport/KFDK

 

I started with setting the EPSG coordinate system - then use geolocate to set internal origin to a point - (I copied the lat and long coords from the corner of airfield on google maps. Then use geoimage to clip out your boundary. In OIP request map tiles and choose appropriate scale. 1:5000.. then draw the line and try measuring.

 

file attached

 

Frederick-MD.vwx

HTH

 

E