Tutorial:Draping field map slips over Ordnance Survey NTF data

Introduction• This folder contains the instructions and data

necessary for draping an example field slip over a DEM.

• The tutorial is based on the Invernaver area, NW Highlands.

• The exercise covers preparing the field map slip image

• Obtaining the DEM• Importing the DEM• Draping the field slip over the DEM

Preparing the field slip: Scanning

• Scan the field slip at approximately 200 DPI and crop off unwanted areas in an appropriate image editor to reduce file size.

• Save the image as a TIFF

Preparing the field slip: Importing

• Firstly the image must be converted to the IMG format used by ERDAS.

• Use the Import dialogue from the main ERDAS toolbar.

• Choose the file type (Tiff –Direct read) navigate to the TIFF file you wish to import and provide an output file name

Preparing the field slip: Georeferencing

• The field slip may now be opened in ERDAS viewer, accessed from the main ERDAS toolbar.

• Correct strange colour schemes (as shown here) by rearranging the red and blue layers in:– Raster band combinations

• Currently the Geo map has no coordinate values associated with it. The following few pages demonstrate adding these values

Georeferencing• The georeferencing dialogue is accessed through: Raster Geometric

correction• A series of boxes must then be completed before georeferencing can begin

Choose Polynomial as a generic geocorrectingmethod

Use this to Add/change a Projection scheme (and Units). For the UK choose British National Grid from one of the standard formats

This box asks where the information is going to be collected from. Usually the viewer with the image currently open

Georeferencing

• Eventually the georeferencing tools boxes will pop up.

• The Ground Control Point (GCP) tools enable you to pick points on the scanned map and assign real world coordinates to the associated file coordinates.

Georeferencing• Use the Geo Correction

Tool (circle with cross at centre) to position markers at locations of a known geographical coordinate (i.e., grid line intersections)

• Always place the markers using the left hand magnified viewer – viewer 3. This magnified view can be dragged around

Georeferencing• Define the geographical

coordinate of a located point in the X ref and Y ref columns.

• You may have to go back and precisely reposition the points to reduce residual and RMS error

Georeferencing• Once you are happy with

the coordinate values and overall error the image can be resampled by clicking on the resample button

• Provide a new name when prompted and click OK, and OK again.

• The image is now georeferenced and should display correctly with a DEM of the same area

Resample button

DEM

• National data Transfer Format (ntf) files available from the Ordnance Survey are not directly importable into ERDAS Imagine.

• This section may become obsolete with the next release of ERDAS.

• The following section describes how to obtain a DEM of a specific area of the UK and import it into ERDAS

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 a

week• Leeds University has a quota for the quantity it is

able to download from this site. In order to preserve download credits try not to duplicate downloads and find out if the resource already exists locally

DEM: Obtaining a file• Two forms of DEM are available, both of which

are associated with the OS topographic mapping series– 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 one

elevation point every 30 m - probably too coarse to be useful

• The DEMs are catalogued in the same way as the OS maps using OS coordinates or map codes. Navigate to the required DEM and download

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

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 the mapping menu in certain ISS clusters)

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.

DEM: Importing• Select a suitable output destination, leave everything

else as default.

• Click the convert button to import the data

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 of the 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

DEM: Importing into ERDAS

• The ascii data file (.asc) may now be imported using the IMPORT dialogue from the main ERDAS toolbar as an ASCII Raster

• Select the file location, a suitable output location and provide a file name– OK

DEM: Importing into ERDAS• Clicking OK brings up an ASCII

import dialogue where the attributes of the file may be defined.

• Specify the number of rows and columns (i.e., 501 for a OS Profile DTM).

• Number of bands remains at 1 (as the data is one band black and white).

• Line terminator must be changed to return to reflect the data format.

• Also change Pixel arrangement to white space (i.e., white space between values in the ASC file).

• Next click Import Options and change Output data type to float. This maintains the decimal places from the original file.

• OK both boxes to begin the import.

Georeferencing the DEM

• The imported IMG file can now be displayed in the ERDAS viewer.

• Unfortunately the georeferencing of the image has not been imported and must be associated with the image in order for it to display correctly with other images

Georeferencing the DEM

• From the Viewer menu choose

Utility layer Info

• Then choose change map model from the Edit menu

Georeferencing the DEM• The coordinates of a corner of the

IMG file may be inputted from the data contained in the ASC files header.

• Note ERDAS requires the Upper Left corner, while the ASC file provides the Lower Left.

• This can be accommodated by adding 5000 meters to the y value (i.e., the north to south width of the DEM tile).

• Define the pixel size and the units used.

• Next define the map projection scheme. For the United Kingdom this is the Transverse Mercator, which the British National Grid is associated with.

Georeferencing the DEM

• After a map model has been chosen the projection scheme can be more precisely defined in Add/ Change projection -accessed through the Image Info Window – Edit Add/Change projection

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

Display problems

• Problems with colour display may again arise from ERDAS displaying the Red, Green and Blue layers in the wrong order.

• Correct in:– Raster band

combinations

Display problems

• Other problems may arise because:• Your DEM and geomap are not of the

same geographical area.• Mistyped coordinate values in either DEM

Map reprojection or the map geo-referencing parts of this exercise. Pay particular attention to correcting the corner values of the DEM, and inputting the right quantity of zeros.