Tuesday, September 09, 2008

Texas Statewide Historical Maps & Positional Accuracy Pt. 2

Continuation from Pt. 1

Our Geographic Accuracy Measurement Procedures - Overview
  • We calculate 5 positional accuracy measurements for each georeferenced map. Texas is divided into 4 quadrants, with each quadrant receiving an independent positional accuracy measurement. The fifth measurement is the average accuracy of the 4 quadrants.
  • Each quadrant is further subdivided into 4 sections. At least 4 sample points are taken from each of these sections, with a minimum of 20 points from each quadrant. However, as the NSSDA suggests, 25 points are recommended and is what we aim for. We therefore end up using (ideally) between 80 to 100 sample data points to measure the accuracy of each map.
    • The exact points we use for each map of course differ, but here is our priority geographic references. First, clearly identifiable county boundaries. Second, coordinates provided by hash marks labeled on map edges. Third, city locations. City locations have the lowest priority here because we use them primarily to georeference and it is best for the sample points to be different from the points used to georeference. Natural features, such as rivers or lakes, are never used.
Our Geographic Accuracy Measurement Procedures - Step by Steps
  • Four new feature classes are created for each map to hold the 20 to 25 sample points from each quadrant.
  • Point features are created for each sample data point. The coordinates as specified by the map are hand-entered in the attribute table.
  • The actual X/Y coords (in meters) are generated for each feature in the four quadrant feature classes using ArcMap's Add XY Coordinates tool.
  • This data is entered into the horizontal accuracy calculation spreadsheet. Our own customized templates use different NSSDA multipliers, based on the calculated RSME ratio. This is discussed in more detail in the previous blog post.
An Example! Enough Talk, Here is an Example...
  • Title: Colton's Texas (1855)
    • Original Map Citation: Colton, Joseph H. Colton's Texas. New York: J. H. Colton & Co., 1861.
    • Entire map tested 8.982 kilometers horizontal accuracy at 95% confidence level.
    • Download: Georeferenced Map - No metadata included yet as we are still ironing out taxonomy and copyright issues. You betcha there will be posting about these issues, as well as interface and file format issues as well.
  • Horizontal Positional Accuracy statement for the Colton's Texas (1855) map is pasted below. As specified above, I am unable to post a complete metadata record at this time as other portions are incomplete. You can see relative high accuracy of NE Texas as opposed to the SE quadrant.


Each georeferenced map image file was divided into four quadrants at centerpoint 31.688, -98.634. The horizontal accuracy of each quadrant was calculated independently using the NSSDA (National Standard for Spatial Accuracy) at the 95% confidence level. Where possible, a minimum of 20 sample points from each quadrant were used to measure horizontal accuracy. The horizontal accuracy of of the entire map is the mean average of these four measurements.

Horizontal_Positional_Accuracy_Value: 8982
Entire map tested 8.982 kilometers horizontal accuracy at 95% confidence level. Mean average of the four quadrants.

Horizontal_Positional_Accuracy_Value: 6988
NW Quadrant tested at 6.988 kilometers at the 95% confidence level.

Twenty-four points were used to test the positional accuracy. The calculated RSME ratio was 0.3344, and the normalized elliptical error table was used to determine error at the 95% confidence level.

Horizontal_Positional_Accuracy_Value: 8100
NE quadrant tested at 8.1 kilometers at the 95% confidence level. Twenty-four points were used to test the positional accuracy. The calculated RSME ratio was 0.699, and the normalized circular error table was used to determine error at the 95% confidence level.

Horizontal_Positional_Accuracy_Value: 12400
SE quadrant tested at 12.4 kilometers at the 95% confidence level.

Twenty-four points were used to test the positional accuracy. The calculated RSME ratio was 0.609, and the normalized circular error table was used to determine error at the 95% confidence level.

Horizontal_Positional_Accuracy_Value: 8400
SW quadrant tested at 8.44 kilometers at the 95% confidence level.

Ten points were used to test the positional accuracy. The calculated RSME ratio was 0.400, and the normalized elliptical error table was used to determine error at the 95% confidence level.


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