Maintaining High Accuracy in Modern Geospatial Data

Transcription

1 Maintaining High Accuracy in Modern Geospatial Data Patrick Cunningham President +1 (207) Copyright 2010 Blue Marble Geographics

2 Concepts Geodesy - Geomatics Geoid Ellipsoid Datum Datum Shift Projection Semi-Major Axis/Semi Minor Axis Prime Meridian Geocentric Translation / Position Vector Frame Rotation Universal Transverse Mercator/Robinson Coordinate Reference System NAD83/Texas North Central (USft) Name Semi-major axis (a) Flattening (f) International m 1/297.0 WGS m 1/ e 2 = a 2 - b 2 C6H12O6 2 CH3CH2OH + 2 CO2 b 2 1 -a z A y R = A 1 -a z -a y A x 1 =a/ (1-e2sin2 ) 0.5 E = FE + [A-TA 3 /6-(8-T+8C)TA 5 /120] k AB =(k AB +4k M +k B ) 2

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4 Coordinate System??? Coordinate Reference System A reference system used to measure horizontal and/or vertical distances and angles on a map. Defined by: Ellipsoid Datum Map Projection Units Origin E.D.P.U.O. 4

5 Mind the Gap Between the World and Map! 5

6 Image Courtesy of Merrick & Co., Inc. 6

7 Geodetic Coordinate Ref. Systems Geodetic Coordinate Systems The Earth is not round, but rather a lumpy pseudosphere. Despite this, globes are the most accurate map of Earth we have. Globes good earth model, without too much distortion Ellipsoid turned into a datum with an origin, all angles All geodetic systems use Latitude and Longitude to define the positions on the Earth. These are angular measurements, not linear. The two main reference lines from which all distances and locations are calculated are the Equator and the Prime Meridian. 7

8 Units The Overlooked Step Child What are Coordinates? A set of numbers used to represent points in space Relative to the Origin and Axes Depending on the coordinate system, coordinates can be displayed in various formats of various units. Units Angular In Geodetic Coordinate Systems, coordinates are given in angular units, the most common being Degrees. Angular units can be displayed in a number of formats Linear Cartesian and Projected coordinate systems use linear measures Unit format is mostly limited to decimal precision. 8

9 Types of Coordinate Ref. Systems Geographic (2d +1) Projected (2d +1) Geocentric or Earth Centered, Earth Fixed (3d) 9

10 Datum Transformations - Examples When coordinates exist on one datum, and the need exists to plot or display them based on another, a datum transformation is required. 3 Parameter shift (Molodensky, Geocentric): When performing a 3 parameter datum transformation, a linear shift is applied to the x, y, and z axes at the geocenter. Accuracy can be +/- 1 to 50 meters. Does not model network distortions well. But, they are easy to calculate! 7 Parameter Shift (Bursa-Wolfe, Helmert): The same x,y,z shift found in the Molodensky method, but with 3 additional parameters of rotations of the axes and a scale. If applied properly can allow for more accurate mapping of +/- 1m over large areas. Rotation sense is critical Other Transformation Methods (High Accuracy): Molodensky-Badekas 10 parameter (10-15cm) NADCON, HARN, NTv2, OSTN02, etc: (7-15cm) 10

11 Transformations Example NAD 27 to NAD 83 NADCON (15cm) DMA Multiple Regression Eq. (1m) Canadian NTv2 (3m) 3 Parameter Eastern US (13m) Continental US (14.5m) 11

12 Early Binding vrs Late Binding World Geodetic System of 1984 is a standard ellipsoid definition that is widely used all over the world. Earth-centered, a best fit for the whole earth Originally created as Geodetic Reference System of 1980 by the US Defense Mapping Agency Used as the basis worldwide for the GPS satellite network It serves as a common ellipsoid/datum through which we convert most other datums NAD27 Timbalai ED50 Minna WGS84? AGD66 Beijing 54 M poraloka PSAD56 12

13 Vertical Datums Horizontal Datum Defines the Earth s curvature over a region or the entire planet Oversimplifies the shape Vertical Datum Adds the lumps back in Models of Mean Sea Level Both types give us a zero surface to base elevations on Surface is irrelevant to ECEF Normal to Ellipsoid Normal to Geoid Ellipsoid Geoid Terrain 13

14 Geoids and Mean Sea Level NOT equal! Vertical models are approximations of Mean Sea Level MSL is a moving target For highest accuracy, local vertical deflections must be taken into account The models are not all the same! Image Courtesy of Seacoos.org 14

15 < 1 Meter Working at high precision levels requires care and persistence The work isn t done when the data has been collected There are: Assumptions (to question) Data Manipulations (to understand) Limitations (to acknowledge) 15

16 +/- 1 Meter - Assumptions EPSG Database states that: WGS84 Transit, WGS84 (G730), WGS84 (G873), WGS84 (G1150), NAD83 (1986), NAD83(NSRS2007), NAD83 (CSRS), ITRF (all epochs), GDA94, RGF95, JGD2000, IRENET95, ETRS89 About 60 datums in all, worldwide are coincident at the +/- 1 meter level Because of this, many applications are set up with that assumption. Because of this, a lot of people are too. 16

17 Where When is your data? If WGS 84 were a person, it would now be old enough to get cheap car insurance. In the 28 years it s been around, the Earth hasn t stopped changing. Your coordinates aren t where you left them. Image courtesy of NGS 17

18 Time Dependant Shifts Horizontal Time Dependant Positioning (HTDP) for North America AUSPOS for Australia 14 parameter transformations for other parts of the world These can be implemented in current software as fixed epoch transforms 18

19 In Sum CRS Transform not convert Types of CRS Types of Datum Transformations Geoids WGS 84 early & late / age HTDP Processes/Assumptions 19

20 Solutions Metadata Vague descriptions lead to assumptions! Education GIS is science, there s a lot to learn! Training Software tools are not always as simple as we would like; you have to know how to use them! Mind the Gap Between the World and Map 20

21 Thank You! Questions? Patrick Cunningham President +1 (207) Copyright 2010 Blue Marble Geographics