erdas - draping field map slips over ordnance survey ntf data
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Tutorial:
Draping field map slips overOrdnance Survey NTF data
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Introduction This folder contains the instructions and data
necessary for draping an example field slip overa DEM.
The tutorial is based on the Invernaver area, NW
Highlands. The exercise covers preparing the field map slipimage
Obtaining the DEM
Importing the DEM
Draping the field slip over the DEM
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Preparing the field slip: Scanning
Scan the field slip at approximately 200
DPI and crop off unwanted areas in anappropriate image editor to reduce file
size.
Save the image as a TIFF
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Preparing the field slip: Importing
Firstly the image must beconverted to the IMG formatused by ERDAS.
Use the Import dialogue fromthe main ERDAS toolbar.
Choose the file type (Tiff Direct read) navigate to the
TIFF file you wish to importand provide an output filename
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Preparing the field slip:
Georeferencing
The field slip may now be
opened in ERDAS viewer,accessed from the mainERDAS toolbar.
Correct strange colourschemes (as shown here) by
rearranging the red and bluelayers in:
Raster band combinations
Currently the Geo map hasno coordinate valuesassociated with it. Thefollowing few pagesdemonstrate adding thesevalues
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Georeferencing The georeferencing dialogue is accessed through: Raster Geometric
correction
A series of boxes must then be completed before georeferencing can begin
ChoosePolynomial as
a generic
geocorrecting
method
Use this to Add/change a
Projection scheme (and
Units). For the UK choose
British National Grid from oneof the standard formats
This box asks where the
information is going to be
collected from. Usually
the viewer with the imagecurrently open
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Georeferencing
Eventually the georeferencing tools boxes
will pop up.
The Ground Control Point (GCP) tools
enable you to pick points on the scannedmap and assign real world coordinates to
the associated file coordinates.
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Georeferencing Use the Geo Correction
Tool (circle with cross at
centre) to position markersat locations of a knowngeographical coordinate(i.e., grid line intersections)
Always place the markers
using the left handmagnified viewer viewer 3.This magnified view can bedragged around
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Georeferencing Define the geographical
coordinate of a locatedpoint in the X ref and Yref columns.
You may have to go
back and preciselyreposition the points toreduce residual andRMS error
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Georeferencing Once you are happy with
the coordinate values andoverall error the imagecan be resampled byclicking on the resamplebutton
Provide a new namewhen prompted and clickOK, and OK again.
The image is nowgeoreferenced andshould display correctlywith a DEM of the samearea
Resample button
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DEM
National data Transfer Format (ntf) files
available from the Ordnance Survey arenot directly importable into ERDASImagine.
This section may become obsolete withthe next release of ERDAS.
The following section describes how to
obtain a DEM of a specific area of the UKand import it into ERDAS
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DEM: Obtaining a file Ordnance Survey mapping files are directly
available using an Athens password The files can be found on the website:
http://digimap.edina.ac.uk/
Registration to use this website takes around aweek
Leeds University has a quota for the quantity it isable to download from this site. In order to
preserve download credits try not to duplicatedownloads and find out if the resource alreadyexists locally
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DEM: Obtaining a file Two forms of DEM are available, both of which
are associated with the OS topographic mappingseries Profile DTM (10m, or 1:10,000) is the version most
useful for the purpose of draping field mapping slips
Panorama DTM (30m or 1:50,000) contain oneelevation point every 30 m - probably too coarse to beuseful
The DEMs are catalogued in the same way as
the OS maps using OS coordinates or mapcodes. Navigate to the required DEM anddownload
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DEM: NTF file format
Ordnance Survey (OS) DTM data is in
.NTF format (a plain text file with header)which is not yet directly readable by
ERDAS.
The following section demonstrates how to
import an OS DEM into ERDAS
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DEM: Importing
Firstly the DEM file must be converted to a
file format readable by ERDAS
This transformation is conducted in the
MapManager program (found under themapping menu in certain ISS clusters)
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DEM: Importing
Select the file to be converted (profile-dtm-nc66se-20030806.ntf) File select files
OK
This brings up series of screens describing the data format and location.
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DEM: Importing Select a suitable output destination, leave everything
else as default.
Click the
convert button
to import thedata
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DEM: Importing Importing through MapManager converts the messy NTF
data into a ASCII Raster grid format (.ASC file) directly
importable by ERDAS (almost!)
The header describes the
number of rows & columns,
and the OS grid coordinates ofthe lower left corner (ll).
The cell size is the grid
spacing (10 x 10 meters).
The grid data below therefore
covers 501 x 10 m in the x and
y directions
The grid data itself consists
entirely of elevation (or z) data
values
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DEM: Importing into ERDAS
The ascii data file (.asc)may now be importedusing the IMPORT
dialogue from the mainERDAS toolbar as anASCII Raster
Select the file location, a
suitable output locationand provide a file name OK
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DEM: Importing into ERDAS Clicking OK brings up an ASCII
import dialogue where the attributesof the file may be defined.
Specify the number of rows andcolumns (i.e., 501 for a OS ProfileDTM).
Number of bands remains at 1 (asthe data is one band black andwhite).
Line terminator must be changed toreturn to reflect the data format.
Also change Pixel arrangement towhite space (i.e., white spacebetween values in the ASC file).
Next click Import Options and
change Output data type to float.This maintains the decimal placesfrom the original file.
OK both boxes to begin the import.
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Georeferencing the DEM
The imported IMG file cannow be displayed in the
ERDAS viewer.
Unfortunately the
georeferencing of the imagehas not been imported and
must be associated with the
image in order for it to display
correctly with other images
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Georeferencing the DEM From the Viewer
menu chooseUtility layer Info
Then choose
change map
model from the
Edit menu
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Georeferencing the DEM The coordinates of a corner of the
IMG file may be inputted from thedata contained in the ASC filesheader.
Note ERDAS requires the UpperLeft corner, while the ASC fileprovides the Lower Left.
This can be accommodated by
adding 5000 meters to the yvalue (i.e., the north to southwidth of the DEM tile).
Define the pixel size and the unitsused.
Next define the map projection
scheme. For the United Kingdomthis is the Transverse Mercator,which the British National Grid isassociated with.
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Georeferencing the DEM
After a map model has been chosen theprojection scheme can be more preciselydefined in Add/ Change projection -accessed through the Image Info Window
Edit Add/Change projection
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Displaying the DEM The georeferenced DEM and overlay geomap may now be correctly
displayed in ERDAS Virtual GIS viewer accessed from the Main
ERDAS toolbar
File Open DEM File Open Raster layer
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Display problems Problems with
colour display mayagain arise from
ERDAS displaying
the Red, Green andBlue layers in the
wrong order.
Correct in: Raster band
combinations
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Display problems Other problems may arise because:
Your DEM and geomap are not of thesame geographical area.
Mistyped coordinate values in either DEM
Map reprojection or the map geo-referencing parts of this exercise. Payparticular attention to correcting the corner
values of the DEM, and inputting the rightquantity of zeros.
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