Low Distortion Projection Tools additions to the Geodetic

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

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.

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

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

LDP Definition Tool 1. User specifies area of interest 2. 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 3. User accepts, or modifies parameters and iterates 4. Upon completion: – a final graphic is provided along with metadata files – Offer to “register” the projection

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

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

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.

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

Questions?