1. 2- مبنای مسطحاتی Datum
What is Datum?
Spheroid
Ellipsoid
Geoid

2. Spheroid
The earth is not a perfect sphere.
True sphere

3. Mastering ArcGIS
Ellipsoid
Chapter 3
Ellipsoid
Actual shape is closer to an ellipsoid then mapping a point onto an ellipsoid better represents its position on the earth’s surface 3

4. زمین بدون اقیانوس و دریاها

5. Geoid
Mastering ArcGIS
Chapter 3
زمین یک بیضوی ایده آل نیز نیست. ژئوئید یک سطح فرضی است که بر پایه اندازه گیری های ثقل سنجی تعیین موقعیت شده است. سطح ژئوئید بسیار نامنظم، ناهموار و پیچیده است بنابراین نمی توان از این سطح به عنوان یک مبنای ریاضی استفاده کرد.
در قسمت هایی از سطح زمین که ژئوئید منطبق بر بیضوی زمین است می توان از بیضوی سطح زمین برای تولید نقشه استفاده نمود. 5

6. Mastering ArcGIS
Chapter 3
Datum
Best fit for North America
To minimize the discrepancy between the geoid and ellipsoid, a datum is defined
A datum shifts the ellipsoid relative to the geoid to achieve a best fit between the two
A local datum optimizes the shift for the best fit at a particular location. It may also use a surveyed network of points to make further adjustments
A geocentric or world-centered datum optimizes the fit for the entire earth
Worse fit for South America
Copyright © 2009 by Maribeth H. Price 6

7. Mastering ArcGIS
summary
Chapter 3
Datum: The best possible fit between the earth geoid and the mapping ellipsoid
Common Datums
North American 1983 (uses GRS80 spheroid)
North American 1927 (Clarke1866 spheroid)
World Geodetic System 1984 or WGS84
Global Reference System 1980 or GRS80
Sphere
Spheroid
Geoid
Datum 7

8. Datum examples Local datums World datums
Mastering ArcGIS
Chapter 3
Datum examples
Local datums
Optimized for a country or continent or local area
May include fitted survey points for additional corrections
World datums
Optimized for the entire globe
Local datums
World datums
Copyright © 2009 by Maribeth H. Price 8

9. Projections and datums
Mastering ArcGIS
Chapter 3
Projections and datums
Every map projection is based on a GCS
Every GCS has a datum
Therefore: Every projection has a datum
Projections based on different datums will be offset from one another
Amount of offset depends on region
Offset typically ranges from 0 – 300 meters
UTM Zone 13 NAD ≠ UTM Zone 13 NAD 1927!
Copyright © 2009 by Maribeth H. Price 9

10. Typical datum offset Roads in NAD83 Photo in NAD27 Mastering ArcGIS
Chapter 3
Typical datum offset
Roads in NAD83
Photo in NAD27
Copyright © 2009 by Maribeth H. Price 10

11. False easting and northing
Mastering ArcGIS
3- جابجایی مرکز مختصات
False easting and northing
Chapter 3
Note the negative coordinate values
(-x,y)
(x,y)
Reference latitude
(0,0)
(-x,-y)
(x,-y)
Central meridian 11

12. False easting and northing
Mastering ArcGIS
Chapter 3
False easting and northing
False easting
= 500,000
(x,y)
(x,y)
Reference latitude
(x,y)
(x,y)
False northing = 500,000
(0,0)
Central meridian
Copyright © 2009 by Maribeth H. Price 12

13. Universal Transverse Mercator UTM

14. Universal Transverse Mercator
Mastering ArcGIS
Chapter 3
Universal Transverse Mercator
Zone 16
South Dakota
Based on Transverse Mercator (cylindrical) projection
World divided into 60 zones 6 degrees wide
Distortion is minimal within each zone
Maps of different areas use best zone
Best for maps covering small area in one zone
Copyright © 2009 by Maribeth H. Price 14

15. Where is the zone 1 ? UTM Zone number=Round(longitude/6)+31
Zone 1 : 180° W ° W
Central meridian : 177°W
UTM Zone number=Round(longitude/6)+31

16. UTM
Easting(X)
Northing(Y)

17. How is UTM domain ?

18. The number 712000mE is read as, “712,000 meters East.”
711 is short-hand for 711,000, and is 1000 meters West of mE.

19. The number 4765000mN is read, “4,765,000 meters north” of the equator.
The next UTM number above is 4766,
and is 1000 meters north.

20. Setting the display units
Mastering ArcGIS
Chapter 3
Setting the display units
Copyright © 2009 by Maribeth H. Price 20