1 Spatial Data Spatial data comes in many forms. So How does a GIS work with the data so that it can put the data in the right place on a map?

2 Outline Kinds of spatial data –Vector data Points, lines, and polygons Highway maps –Raster data and image data Raster representation of data Satellite data (Landsat) Aerial photographs Digital elevation data Real world problems associated converting data from earth’s spherical coordinates to coordinates on a flat surface

324 3 Compare Raster & Vector Vector GIS –Objects represented by: points lines polygons large database each object Raster GIS –AREA represented by: Grid cells one value per cell Large number thematic layers Forest road Cropland stream Looks like a map Looks like what?

4 Vector Data Line Area Point “Kinds” of GI data Aerial Photo Landsat 7 image 2’ resolution rectified aerial photograph

5 The Digital Elevation Raster

6 Uses Vector data is most common because you can tie huge databases to features BUT Raster data is very good for continuous surfaces like –Elevation Images (dumb picture) data

7 OK… Data on the globe is not very useful because you can’t put the globe in your report So you need a 2D map Thus you have to convert 3D data to 2D Lets start with the spherical earth …

8 Syracuse W 43.07N  Where are we?

9 Earth’s Coordinate System X is Longitude and is measured E and W from Greenwich, England. West is negative, East is positive Y is latitude and is measured N and S from the equator. North is positive and S is negative. These are called Geographic Coordinates

10 X, Y = Longitude, Latitude Lines of constant Longitude Lines of constant Latitude Equator Stretch the top Stretch the bottom

11 X, Y = Longitude, Latitude Lines of constant Longitude Lines of constant Latitude Equator 90E, 30N 90W, 30S +90, W76.57° N42.93°-76.57° 42.93°

12 The world in Geographic Coordinates Is Antarctica Really that big?

13 3D to 2D Geographic coordinates introduce too much distortion to be useful So we need to convert 3D coordinates into 2D coordinates But, there is a problem…

14 The Problem

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17 The Mercator Projection Making a Projection

18 Some Projections

19 Some Projections It is pretty obvious that if you have data in different projections they are NOT going to “line up” with each other

20 Projections: Distortion In going from spherical coordinates (surface) to a flat surface THERE WILL BE DISTORTIONS of one or more of the following –Shape –Area –Distance –Direction

21 Projections: Distortion In going from spherical coordinates (surface) to a flat surface THERE WILL BE DISTORTIONS of one or more of the following –Shape –Area –Distance –Direction

22 That means… Data in different projections will not line up or be congruent! This is something you have to be aware of … HOWEVER ArcGIS will project on the fly so the problem is not great But only IF there is a metadata file for the data. Data about Data

23 Projections: Distortion Shape: If shapes look the same on the map and on the globe then the projection is conformal Area: If area is preserved then you have an equal area map Distance: If distance is preserved then the map is of uniform scale and you have an equidistance map. Direction: maps If directions from a central location to all other points are correct then the map is Azmuthal

24 Projections: Distortion Shape: If shapes look the same on the map and on the globe then the projection is conformal Area: If area is preserved then you have an equal area map Distance: If distance is preserved then the map is of uniform scale and you have an equidistance map. Direction: maps If directions from a central location to all other points are correct then the map is Azmuthal

25 Lets make life a bit more difficult In addition to the many projections that 2D data can be in… There are two Coordinate Systems that are in common use … For smaller areas (like ½ a state) Much of the data you will find useful will be in one of these systems

26 These systems are… The UTM coordinate system or Universal Transverse Mercator coordinate system And The State Plane coordinate system –Unique to each state

27 UTM Coordinate Systems The UTM Coordinate system is – –based on the Mercator projection –A world wide system Except that the cylinder is now horizontal and so is tangent to the earth along a meridian which passes through the Poles Central Meridian Errors are Zero!

28 UTM coordinate system Is a projected coordinate system that divides the world into 60 north and south zones, each six degrees wide. Why bother? Increase Accuracy and decrease distortion Because all the data for a zone is within 3 degrees of the Central meridian it is pretty accurate! Can’t map within multiple zones New York is usually mapped in one zone

29 UTM Zones Most of NY is in UTM Zone 18

30 UTM Coordinates Easting(X) Northing(Y) The units in UTM are usually Meters The coordinates are Eastings & Northings The zone has to be specified Example: Location of CCC is: 373,800 Meters E & 4,756,000 Meters N in Zone 18, N

31 The State Plane Coordinate System A projected coordinate system used in the United States Divides each state into one or more zones Also known as SPCS and SPC.

32 State Plane Horizontal zones (Tenn) are in Lambert Conformal projections Vertical zones are in Transverse Mercator projections Each state has its own origins for its own system States may have multiple zones in different projections UNITS are usually feet BUT NOT ALWAYS

33 State Plane Zones NY West Zone 4851 NY Central Zone 4826 NY East Zone 4801 NY Long Island Zone 4876

34 State Plane Zones NY West Zone 4851 NY Central Zone 4826 NY East Zone 4801 NY Long Island Zone 4876 Transverse Mercator Lambert Conformal

35 ArcMap Problem (or NOT) ArcGIS will project on-the-fly By that, we mean that if you add a layer that is NOT in the same Coordinate System, Projection, etc. as the data in the map ArcMap will project (verb) the new data to match that of the data already loaded Is there a problem?? Data has to have a metadata file

36 Another niggling Problem The earth is only approximately spherical We can mathematically convert features on the 3D earth to a 2D map easily if the surface is spherical and smooth Oops - earth is pear shaped and rough So we have to introduce the idea of a datum

37 Earth Spheroids & Datums A spheroid can be moved mathematically to fit different parts of the earth… FIT Fit Spheroid Now we have 2 different datums

38 So what? The spatial properties of a GIS data layer specify both the projection or Coordinate system and the Datum Different datums will cause shifts in location of the order of 100 meters Not big but troublesome In ArcGIS on-the-fly projection takes care of both projection and datum

39 BUT… This neat functionality of ArcGIS only works if you have a metadata file for each layer Sometimes that is a problem!

40 Some Datums These are the common datums For Coordinate Systems the spatial properties are given in statements like… NAD 27NAD 83WGS 84 NAD_27_UTM _ZONE_18N NAD_83_SPC _ZONE_4826

41 Summary so far 1.There are a number of GIS data types 2.Spherical earth (globe) a.Longitude, Latitude (X,Y) 3.Flat maps a.Projections b.Coordinate Systems UTM SP Spheroids & Datums 4.There is one other factor influencing data accuracy - scale

42 Scale Scale = distance on map distance on ground A Scale of 1/24,000 Means 1 inch (or foot, or furlong) on the map = 24,000 inches (or feet or furlongs) on the ground.

mi 3.9 Numeric Scale = 1/5,198,769 New York

Main St. 3.6” 2.63mi Scale = 1/46,288

45 Living Room Kitchen Dining Room 2.6” / 25’ Scale = 1/115

46 Living Room Kitchen Dinning Rm. Scale 1/47,000,000 1/46,000 1/115 Is a smaller number than Small Scale data Large area/sheet Least accurate Large Scale data Small area / sheet Most accurate

47 So what’s all the fuss? Scale is a very important property of maps and digital data derived from maps. Why? Because it stands for accuracy in the data A small scale map is less accurate than a large scale map. Why is that? Generalization

48 Accuracy & Generalization Take the case of a winding stream Shrink it to a Smaller scale Now it is hard to see what is there So the cartographer simplifies the stream

49 Summary There are a variety of spatial data types Spherical Geographic Coordinate Systems are based on Spheroids Spherical data is projected onto 2D maps There are many Projections More commonly, you will run into the class of Projections called Coordinate Systems (UTM, SP) Projected data is based on a datum and data in different datums will not (usually) line up!

50 Summary The subject of projections and datums is the most confusing and complex area of using GIS. Take good notes and do your best to understand it. At GIS conferences sessions on this topic are always very crowded! That tells you something!

51 Appendix These slides will not be projected and are provided as a resource

52 Acronyms NAD – North American datum GCS – Geographic Coordinate System WGS – World Geodetic System UTM – Universal Transverse Mercator corrdinate sysem SPC – State Plane coordinate system GRS –Geodetic Reference System DD – Decimal Degrees DMS – Degrees, minutes, seconds HARN – High Accuracy Reference Network (State Level) NADCON – North American Datum Conversion between NAD27 & 83

53 Definitions Vector data. Data made up of points, lines, and areas or polygons. Raster data, Data which represents the earth’s surface and/or what is on it as a collection of cells, usually square. Longitude – The X in spherical coordinates Latitude – The Y ins spherical coordinates Meridian – Great Circle passing through both poles Eastings and Northings – X and Y in UTM & SPC