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GPS Mapping Software - Import Raw Image or Elevation Data - Tutorial

Introduction

In GIS, raster data is often exchange as raw data between different GIS software. The Eye4Software GPS Mapping Studio offers import and export functions for raw image or elevation data. There are 3 possible formats: BIL (Band Interleaved by Line), BIP (Band Interleaved by Pixel) or BSQ (Band Sequential). Raw image data is always uncompressed, so the size in bytes is always equal to:

nBytes = nBands x nDataSize x nRows x nCols

Most image data uses one (elevation or colormap data) or three (RGB values) bands. When this kind of data is exported, most software also generates a header and a world file. These files are needed to instruct the reading software how the image data is formatted. Some data sources offer this data without header or world file. Using the Eye4Software GPS Mapping Studio, you can still read these files by entering this information manually.

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Importing the data

To start importing raw image data, open the "Import Map Dialog" by selecting "Import" => "Import Map" from the "File" menu. A file selection dialog will appear. Now set the file filter to "Raw Binary Grid File (*.bil)(*.bip)(*.bsq)(*.dem)" and browse for the file you want to import.

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Either the header or world file is missing

When either the header file (*.hdr) or world file (*.blw, *.bpw or *.bqw) is missing, the following dialog will appear:

  

The software will try to detect as much information as possible. You have to complete the missing information in order to load the map correctly.

Number of rows

The number of pixel rows in the image file. Also known as Y-size, height or length of the image.

Number of columns

The number of pixel columns in the image file. Also known as X-size or width of the image.

No Data Value

This values is only used when the image data contains elevation or depth values. When a pixel value is equal to the "NO DATA" value, it should not be drawn. Values often used are -32768, 32767 or -9999.

Number of bands

When the file contains elevation data, or has a colormap file (*.clr), it is most likely it contains only one single band. True color images or images with transparency usualy have three or four bands.

Bytes per pixel

Most images with 1 band are 2 bytes per pixel (16 bit), except when there is a colormap present. In this case you have to set this value to 1 byte per pixel.

Pixel layout

Defines the format of the file. This probably the same as the extension of the filename.

Byte Order

Ignore when the file contains 1 byte values. If not, and the file was generated on an Intel platform, set to Intel, otherwise, choose Motorola.

After setting the required parameters, the actual file size and calculated file size should match. This check uses the formula mentioned earlier at the top of this page.

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Some examples

The table below shows the settings to use for some known filetypes. If you suspect the data to be imported is generated from a dataformat below, you can copy the settings from the table.

File Type	Rows	Cols	Bands	Bytes	Format	NODATA	Order
SRTM1 files	1201	1201	1	2	BIL	-32768	INTEL
SRTM3 files	3601	3601	1	2	BIL	-32768	INTEL
GTOPO30 files	6000	4800	1	2	BIL	-9999	MOTOROLA
HydroSHEDS files	6000	4800	1	4	BIL	-2147483647	INTEL
ETOPO2	10801	5401	1	2	BIL	-2147483647	INTEL
ETOPO2	10801	5401	1	2	BIL	-2147483647	MOTOROLA
ETOPO2	10801	5401	1	4	BIL	-2147483647	INTEL
ETOPO2	10801	5401	1	4	BIL	-2147483647	MOTOROLA

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