ERDAS - Draping Field Map Slips Over Ordnance Survey NTF Data

7/29/2019 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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ERDAS - Draping Field Map Slips Over Ordnance Survey NTF Data

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