Creating an Event Theme from X, Y Data

Transcription

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2 Creating an Event Theme from X, Y Data In Universal Transverse Mercator (UTM) Coordinates Eastings (measured in meters) typically have 6 digits left of the decimal. Northings (also in meters) typically have 7 digits left of the decimal. Thus, even if no header info is given, you can tell which is which , , , , , , , , , , UTM eastings (X) and northings (Y) for 10 POINT locations collected via GoogleEarth or ArcGIS. Type/pasted into simple TXT editor (comma delimited) and save FILE.txt

3 Open With Excel Open FILE.txt with Excel by changing the input file search to Text Files Save file as an Excel Workbook file (.xls). Add a header row & attribute columns that you want for each point

4 Open Excel in ArcMAP Select Display XY Data

5 Open Excel in ArcMAP Arc first asks which of the 4 numerical attributes fields listed is the X Field and which is the Y Field X & Y Fields default to the first attribute in the Excel file, which happens to be UTM-X Must tell ARC what the projection is. Otherwise, it will be <Undefined> Need to select correct attribute fields for coords.

6 Open Excel in ArcMAP That s OK, ignore this for now

7 Open Excel in ArcMAP Right Click Once your POINT data are displayed in ArcMAP, you can then save them as a shapefile (.shp)

8 Open Excel in ArcMAP

9 Transforming Between Coordinate Systems Necessary if 1) One acquires old feature data (shapefiles) and wishes to integrate these data into a new, ongoing project using a different datum or projection. 2) If one digitizes or scans paper base maps in an old or inconvenient projection. 3) If current projection is not optimal for the map s extent (Lambert CC Chile) Examples include: WHY BOTHER? Transform Datum: NAD27 NAD83 Ellipsoid: Clarke 1866 GRS80 Projection: UTM UTM Datum: NAD27 NAD83 Ellipsoid: Clarke 1866 GRS80 Projection: Geographic (Lat/Lon) UTM Datum: NAD83 NAD83 Ellipsoid: GRS80 GRS80 Projection: Geographic UTM Datum: WGS84 NAD83 Ellipsoid: WGS84 GRS80 Projection: UTM Albers

10 Transforming Between Coordinate Systems In this example, I we will transform from: UTM zone 15 Geographic (Lat/Lon) NAD83 NAD83 GRS80 GRS80 Define the output Coordinate system

11 Transforming Between Coordinate Systems

12 Transforming Between Coordinate Systems At this point, ARC does the transformation, and when it finishes, it automatically loads it into the data view.which is BAD!! Because, these are two totally different coordinate systems. So, one has to remove the UTM layer and then right click on the GEO layer and select Zoom to Layer

13 Export Layer/Shapefile to KMZ file Double clicking this file will invoke GoogleEarth, which zooms to the location of the points. At this, time you can change symbology as you wish.

14 Export Layer/Shapefile to KMZ file

15 Georeferencing an Image By establishing Ground Control Points using a GPS receiver for readily identifiable features (road intersections etc.) We can use ArcMAP s Georeferencing function to rectify/register the image to Earth coords. Is this photo tilted? Gradual topographic variation? Can you rely on photo-measured bearings or azimuths?

16 Georeferencing an Image: An Example I traveled around ISU colleting GCPs using a Trimble GPS. I marked the location of each GCP on the photo while in the field (yellow dots). I then scanned the image and saved it as a JPG file. Notice the image is a little crooked, but that doesn t matter! GCP = Ground Control Point

17 Georeferencing an Image First display the GCP point file Then, the scanned image file Notice you can t see the image This is because it has no real coordinates. This is unlike the images you have downloaded in the past that have coords, but may be missing projection info. Go to Georeferencing tool and select Fit to Display Now, you can see the raw image and the misaligned GCPs.

18 Georeferencing an Image Now we link the GCP s in the POINT file to the yellow points I marked on the map. Under Georeferencing, select Add Control Points icon and start linking marked points to the corresponding GCP After the first link is made the whole image shifts so that the first field mark is now directly over the corresponding GCP The rest are still way off After the second link is made, the whole image is squeezed (or stretched) to fit these two point locations the fit is still poor. keep going, 8 more! Each time the fit is a little better. 2 nd Click 1 st Click

19 Georeferencing an Image Click and drag it over the point.unclick to zoom in, and then resume work

20 Georeferencing an Image After all links are made, the image is now oriented correctly and has the correct aspect ratio (shape). Now, check to see how good the fit is by using the View Link Table icon What is RMS error (RMSE)?

21 Root Mean Squared Error RMSE Residual ERROR (Estimated True) (ERROR) RMSE = RMSE = error 2 N = 6.062m ArcMAP Georeferencing Tool Example Squared = = m

22 Georeferencing an Image 3 rd order polynomial fit RMSE = WOW!! What do you think went wrong? Minimum number or GCPs = (t + 1)(t + 2) 2 t = order of transformation 1 st order = 3 min 2 nd order = 6 min 3 rd order = 10 min We had the min 10 points, but distribution was bad!!

23 Final Task Rectify When your RMSE looks good, galvanize this fit by clicking. Update Georeferencing and then Rectify which invokes the Save As window to set up rectification options

24 DONE!

25 Wednesday

26 Distortion & Aerial Photography Why are aerial photographs prone to displacement propblems? That is, what one thing causes them to misrepresent true ground positions? Remember the vertical pencil example..

27 To Refresh Your Memory on Displacement. Measurements based on Photo # 30 Scale 1:24100 H = Suburb NADIR E = h sub = NADIR E = River Displacement: d = rh H E = h riv = r riv nadir to river * 0.2 = 2.63 r sub nadir to suburb * 0.2 = 3.73 Actual Actual d riv = r riv h riv H nadir = 2.63" ( ) = " Correct map position (GD = 8.75 ) farther away from nadir Displacement d sub = r sub h sub H nadir = 3.73" ( ) Correct map position (GD= 37 ) closer to nadir = "

28 Scale Change is Proportional to Flying Height Terrain height at NADIR is often known. DEM used to visualize scale variation elsewhere. Camera Focal Length (CFL) f = 8.25 inch feet ELEVATION (E) ABOVE MSL (Feet) Flying Altitude above MSL A = feet 250 x 250 meter grid of DEM data PSR PSR = = (A (A-E) - / f / = f = (when E = 0) (when E = 0) Multiple Point Scales in one Photo (Assuming vertical photography) 1 : 15,315 H = A-E 1 : 15,239 A = altitude E = terrain elev

29 National Map Accuracy Standards (NMAS) Initiated in 1937 by American Society of Photogrammetry & Remote Sensing NMAS adopted by U.S. Bureau of the Budget in 1941 Ensures accuracy of both location & elevation for all Federal agencies that produce map products. For maps on publication scales larger than 1:20,000, no more than 10% of points tested can be in error by more than 1/30 th of an inch (0.033 ). Smaller than 1:20,000, this is set to 1/50 th of an inch (0.02 ). For USGS 7.5-minute topoquads (1:24,000): HORIZONTAL 90% of all points tested must be within 1/50 th of an inch (0.05 cm) GD = 40 feet VERTICAL 90% of all points tested must be within ½ a contour interval (contours = 10 ) 5 feet All Federall puplished maps will state: This map complies with National Map Accuracy Standards, and whether it is an enlargement from a particular scale.

30 Estimating Horizontal Map Accuracy on a Photo Say you have a vertical, aerial photograph and you wish to know if you can get away with treating the photo like a map. That is, will photo measurements conform to some expected level of accuracy (e.g., project-level or NMAS)? Example: Frame is a 9 x 9 CIR photo PSR = 25600; flying height (H) = 12,800 ; focal length (f) = 0.5 You know the greatest difference in elevation (h) is ~10 ft. above & 160 ft. below PP. Largest single-photo parallax (r), or distance from PP to some hill/valley, is ~4 For an elevation difference of 160 ft. displacement is: d" = rh H d = 4" = 0.05 At a PSR of 25600, that is a GD error of: 0.05" " = /50 th of an inch = /12 = Should not do it! Impact on area calculations would be: PSR = = 25, 280 PSR = = 24, acres/square inch VS acres/square inch

31 Photo Perspective Has Problems Position Displacement & Scale Variation Tilt (camera/aircraft) Rarely occurs in just one direction (commonly both X & Y) The farther the object is away from the camera the greater the displacement. Scale change is uniform in direction of tilt. Scales constant along lines parallel to tilt. Terrain/Topography Effects are similar to tilt Ground is tilted instead of the camera Displacement effects are directly proportional to terrain complexity In Iowa, terrain change is generally slight & gradual easier to account for (model) by simple rectification using evenly distributed Ground Control Points (GCPs) per photo. As terrain becomes more complex, it is nearly impossible to collect enough GCPs to model (1 st, 2 nd, 3 rd order ) the effects of displacement on true pixel locations. Need a different process.

32 Photo Perspective vs. Orthographic Perspective Single-point Photo Perspective Orthographic Perspective? Variable scale & displacement Constant scale & no displacement

33 Digital Orthophotos Conventional aerial photographs contain image distortions caused by the tilting of the camera and terrain relief. The process of orthorectification removes these distortions and creates an orthophoto an image that looks like an aerial photograph yet has the uniform scale and planimetric accuracy of a map. The first orhtophotos were produce in California in 1965 using an analog process basically triangulation from multiple perspectives. Find true positions for numerous positions via triangulation from 2 or more photos, then reproject onto a new negative. T-64 Orthophotoscope, first instrument used for analog production of orthophotos

34 Digital Orthophotos Triangulation Process Original Image of a roof and the building s base in red Multiple views enable triangulation of true position of roof outline Corrected orthoimage of roof top position over its base

35 Aerial Photograph vs. Orthophoto

36 Orthorectification Using a Digital Elevation Model (DEM) Collecting some ground control points to account for simple 1 st or 2 nd order misalignments to Earth coordinates is a relatively simple task. Modern orthorectification requires use of a detailed DEM and sophisticated software If flying height above terrain (H), camera focal length (f), and aircraft pitch, roll, and yaw amounts are known, then these parameters can be used with a DEM to accurately model photo displacements using triangular irregular networks (TINs). Resulting displacement models producing thousands of uniformly distributed GCPs for the unrectified photo. Transforms photo from single point perspective to multi-point perspective ORTHOGRAPHIC perspective.

37 Microsoft s Global Ortho Program UltraCam-G Sensor flown at H = 5000 m (16,400 ft.) Automated Color Balancing Before After Minimum absolute positional accuracy requirements by land class. Specular Reflection Automated Removal Before After Ortho production then combines this with a detailed, LIDAR-based, DEM to correct tilt & terrain distortions.

38 Digital Orthophotos The Key Points to Remember What is the difference between an aerial photograph and an orthophoto? A conventional perspective aerial photograph contains image displacements caused by camera tilt and variable terrain Aerial photos do not have a uniform scale. Hence, you cannot measure distances reliably on an aerial photograph like you can on a map. Vertical aerial photos (single-point perspective) are not maps. An orthophoto is a uniform-scale photograph..it is a photographic map, and measurements can be made on it like other maps. Therefore, orthophotos may serve as base maps onto which other map information may be overlaid or derived.

39 Orthophoto Design Considerations End users of digital orthophotos have very robust hardware & sophisticated software to view and manipulate orthophoto images. Requires the ability to view selected image features & perform analysis such as relative distance, area, change analysis etc. To meet these demands, proper design of an orthophoto is imperative. Design should consider the following factors: 1. Expected use of the orthophotos & smallest feature to be analyses Urban feature mapping vs. forest cover type mapping 2. Accuracy requirements (relative and feature) X,Y ground distance vs. feature area preservation 3. Equipment, data, and processes used to generate the orthophotos Camera vs. sensor image vs. DEM resolution Transformation/rectification processes used

40 Scanning Images in RM Click START icon Click 1 st option Type in SCAN

41 Scanning Images in RM 241

42 Scanning Images in RM 241