Low Distortion Projection Tools – additions to the Geodetic Tool Kit

1. Low Distortion Projection Tools – additions to the Geodetic Tool Kit Brought to you by Dave Minkel and Michael Dennis

2. Low Distortion Projections – what are we talking about? • A mapping projection that minimizes the difference between distances depicted in a GIS when compared to the real-world distances “at ground”. • “Standard” mapping projections are “at sea level” (ellipsoid), elevation increases the distortion • Flagstaff, AZ (ellipsoid ht ~ 7000 ft) • SPC Distortion = ~ 1:2,300 or -2.3 ft per mile • Phoenix, AZ (ellipsoid ht ~ 1000 ft.) • SPC Distortion = ~ 1:6,800 or -0.8 ft per mile • “Standard” mapping projections usually do not have Central Meridian and Latitude origin near project, which increases distortion variability and convergence angle.

3. Cartoon: Distortion due to change in Earth curvature (1 of 2)

4. LDPs – Who wants them and why? • Engineers & Surveyors use them daily • The value of a GIS increases directly as a function of its accurate portrayal of items of interest • Local govt. GIS managers are realizing the benefits of incorporating as-builts and COGO • Better decision support from the GIS • There is virtually no “cost” to using them • “On-the-fly” reprojection is a reality • Standard Projections are not good enough for local GIS • UTM distortion is 1:2,500 (2.1 ft per mile) • SPC distortion is 1:10,000 (0.5 ft per mile) • But in both cases distortion at ground usually much greater

5. LDP Definition Tool • User specifies area of interest • LDP Tool: • Determines projection parameters • Utilizes USGS National Elevation Dataset and NGS Geoid Model to: • Determine a representative ellipsoid height • Generate a distortion contour plot • Displays distortion plot to user • User accepts, or modifies parameters and iterates • Upon completion: • a final graphic is provided along with metadata files • Offer to “register” the projection

6. Projection “Registry” • A single, national source for the projection parameters of participating local governments • Registration accomplished via • LDP Tool • Web page • Emergency Responders access the Registry through two means: • Subscription – push technology gives them instant updates • Web page – 24 hour, publicly accessible web site

7. So, Why is NGS doing this? Factoids (subject to debate, but not here and now): • The fundamental purpose of the NSRS is to support mapping. • SPC is not adequate for many applications • Use of known (registered), well-defined projections can encourage use of the NSRS rather than local control. • OPUS option for positions in LDP coordinates • Data sheets could include LDP coordinates

8. So, Why is NGS doing this? Factoids (subject to debate, but still not here and now): • The best mapping data for emergency responders is local data. • More current & more accurate • Local GIS will increasingly use LDPs for their data; it will become more difficult to use local data for emergency response without a coordinating activity.

9. So, Why is NGS doing this? • The projection provided is technically the “best” • The LDP tool encourages state and local government to utilize the NSRS as their GIS basis. • The projection is well documented, therefore, it is easy to go between NSRS and a local projection • The Projection Registry mitigates the negative effects of local projections (good and bad). • “Push” technology used to keep ER agencies (i.e. FEMA, DHS, etc.) always up to date. • Web page(s) allows non-subscribers to access Registry information 24/7 • NGS gains VISIBILITY

10. Questions?