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)

Linear distortion due to Earth curvature

Grid length greaterthan ellipsoidal length

(distortion > 0)

Ellipsoidsurface

Projectionsurface(secant)

Maximum projection zone width for balanced positive

and negative distortion

Grid length less thanellipsoidal length

(distortion < 0)

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 user3. User accepts, or modifies parameters and iterates4. Upon completion:

– a final graphic is provided along with metadata files – Offer to “register” the projection

Grid distance less than"ground" distance(distortion < 0)

Linear distortion due to ground height above ellipsoid

Horizontal distance betweenpoints on the ground

(at average height)

Ground surfacein project area

Localprojectionsurface

Ellipsoidsurface

Grid distancegreater than

"ground" distance(distortion > 0)

Typical published "secant" projection

surface (e.g., State Plane, UTM)

Distortion < 0for almost all cases

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?