diff --git a/src/map/IMG/img.cpp b/src/map/IMG/img.cpp
index cdff3e6c..c4f498b6 100644
--- a/src/map/IMG/img.cpp
+++ b/src/map/IMG/img.cpp
@@ -182,7 +182,7 @@ IMG::IMG(const QString &fileName) : _fileName(fileName)
 		_valid = true;
 }
 
-qint64 IMG::read(QFile &file, char *data, qint64 maxSize)
+qint64 IMG::read(QFile &file, char *data, qint64 maxSize) const
 {
 	qint64 ret = file.read(data, maxSize);
 	if (_key)
@@ -191,7 +191,7 @@ qint64 IMG::read(QFile &file, char *data, qint64 maxSize)
 	return ret;
 }
 
-template<class T> bool IMG::readValue(QFile &file, T &val)
+template<class T> bool IMG::readValue(QFile &file, T &val) const
 {
 	T data;
 
@@ -203,7 +203,7 @@ template<class T> bool IMG::readValue(QFile &file, T &val)
 	return true;
 }
 
-bool IMG::readBlock(QFile &file, int blockNum, char *data)
+bool IMG::readBlock(QFile &file, int blockNum, char *data) const
 {
 	if (!file.seek((quint64)blockNum << _blockBits))
 		return false;
diff --git a/src/map/IMG/img.h b/src/map/IMG/img.h
index fd6a65e1..d4b31415 100644
--- a/src/map/IMG/img.h
+++ b/src/map/IMG/img.h
@@ -12,13 +12,12 @@ public:
 
 	const QString &fileName() const {return _fileName;}
 
-private:
-	friend class SubFile;
-
 	unsigned blockBits() const {return _blockBits;}
-	bool readBlock(QFile &file, int blockNum, char *data);
-	qint64 read(QFile &file, char *data, qint64 maxSize);
-	template<class T> bool readValue(QFile &file, T &val);
+	bool readBlock(QFile &file, int blockNum, char *data) const;
+
+private:
+	qint64 read(QFile &file, char *data, qint64 maxSize) const;
+	template<class T> bool readValue(QFile &file, T &val) const;
 
 	QString _fileName;
 	quint8 _key;
diff --git a/src/map/IMG/lblfile.h b/src/map/IMG/lblfile.h
index 48a3dfa5..0db69b44 100644
--- a/src/map/IMG/lblfile.h
+++ b/src/map/IMG/lblfile.h
@@ -12,7 +12,7 @@ class RGNFile;
 class LBLFile : public SubFile
 {
 public:
-	LBLFile(IMG *img)
+	LBLFile(const IMG *img)
 	  : SubFile(img), _huffmanText(0), _table(0), _offset(0), _size(0),
 	  _poiOffset(0), _poiSize(0), _imgOffsetIdSize(0), _poiMultiplier(0),
 	  _multiplier(0), _encoding(0) {}
@@ -20,7 +20,7 @@ public:
 	  : SubFile(path), _huffmanText(0), _table(0), _offset(0), _size(0),
 	  _poiOffset(0), _poiSize(0), _imgOffsetIdSize(0), _poiMultiplier(0),
 	  _multiplier(0), _encoding(0) {}
-	LBLFile(SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
+	LBLFile(const SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
 	  _huffmanText(0), _table(0), _offset(0), _size(0), _poiOffset(0),
 	  _poiSize(0), _imgOffsetIdSize(0), _poiMultiplier(0), _multiplier(0),
 	  _encoding(0) {}
diff --git a/src/map/IMG/netfile.h b/src/map/IMG/netfile.h
index 4a85bb1d..2b11fb5c 100644
--- a/src/map/IMG/netfile.h
+++ b/src/map/IMG/netfile.h
@@ -12,12 +12,12 @@ class HuffmanTable;
 class NETFile : public SubFile
 {
 public:
-	NETFile(IMG *img) : SubFile(img), _huffmanTable(0), _tp(0), _offset(0),
+	NETFile(const IMG *img) : SubFile(img), _huffmanTable(0), _tp(0), _offset(0),
 	  _size(0), _linksOffset(0), _linksSize(0), _shift(0), _linksShift(0) {}
 	NETFile(const QString *path) : SubFile(path), _huffmanTable(0), _tp(0),
 	  _offset(0), _size(0), _linksOffset(0), _linksSize(0), _shift(0),
 	  _linksShift(0) {}
-	NETFile(SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
+	NETFile(const SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
 	  _huffmanTable(0), _tp(0), _offset(0), _size(0), _linksOffset(0),
 	  _linksSize(0), _shift(0), _linksShift(0) {}
 	~NETFile();
diff --git a/src/map/IMG/nodfile.h b/src/map/IMG/nodfile.h
index 4495a873..737e201e 100644
--- a/src/map/IMG/nodfile.h
+++ b/src/map/IMG/nodfile.h
@@ -53,13 +53,13 @@ public:
 		bool eog;
 	};
 
-	NODFile(IMG *img) : SubFile(img), _indexOffset(0), _indexSize(0),
+	NODFile(const IMG *img) : SubFile(img), _indexOffset(0), _indexSize(0),
 	  _indexFlags(0), _blockOffset(0), _blockSize(0), _indexRecordSize(0),
 	  _blockRecordSize(0), _blockShift(0), _nodeShift(0), _indexIdSize(0) {}
 	NODFile(const QString *path) : SubFile(path), _indexOffset(0), _indexSize(0),
 	  _indexFlags(0), _blockOffset(0), _blockSize(0), _indexRecordSize(0),
 	  _blockRecordSize(0), _blockShift(0), _nodeShift(0), _indexIdSize(0) {}
-	NODFile(SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
+	NODFile(const SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
 	  _indexOffset(0), _indexSize(0), _indexFlags(0), _blockOffset(0),
 	  _blockSize(0), _indexRecordSize(0), _blockRecordSize(0), _blockShift(0),
 	  _nodeShift(0), _indexIdSize(0) {}
diff --git a/src/map/IMG/rgnfile.h b/src/map/IMG/rgnfile.h
index 051813a2..80066cd1 100644
--- a/src/map/IMG/rgnfile.h
+++ b/src/map/IMG/rgnfile.h
@@ -20,7 +20,7 @@ public:
 		RoadReference = 0x10
 	};
 
-	RGNFile(IMG *img)
+	RGNFile(const IMG *img)
 	  : SubFile(img), _huffmanTable(0), _offset(0), _size(0), _polygonsOffset(0),
 	  _polygonsSize(0), _linesOffset(0), _linesSize(0), _pointsOffset(0),
 	  _pointsSize(0) {}
@@ -28,7 +28,7 @@ public:
 	  : SubFile(path), _huffmanTable(0), _offset(0), _size(0), _polygonsOffset(0),
 	  _polygonsSize(0), _linesOffset(0), _linesSize(0), _pointsOffset(0),
 	  _pointsSize(0) {}
-	RGNFile(SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
+	RGNFile(const SubFile *gmp, quint32 offset) : SubFile(gmp, offset),
 	  _huffmanTable(0), _offset(0), _size(0), _polygonsOffset(0),
 	  _polygonsSize(0), _linesOffset(0), _linesSize(0), _pointsOffset(0),
 	  _pointsSize(0) {}
diff --git a/src/map/IMG/subfile.h b/src/map/IMG/subfile.h
index 0b67cf88..77e2cf92 100644
--- a/src/map/IMG/subfile.h
+++ b/src/map/IMG/subfile.h
@@ -43,10 +43,10 @@ public:
 		int _pos;
 	};
 
-	SubFile(IMG *img)
+	SubFile(const IMG *img)
 	  : _gmpOffset(0), _img(img), _blocks(new QVector<quint16>()), _path(0) {}
-	SubFile(SubFile *gmp, quint32 offset) : _gmpOffset(offset), _img(gmp->_img),
-	  _blocks(gmp->_blocks), _path(gmp->_path) {}
+	SubFile(const SubFile *gmp, quint32 offset) : _gmpOffset(offset),
+	  _img(gmp->_img), _blocks(gmp->_blocks), _path(gmp->_path) {}
 	SubFile(const QString *path)
 	  : _gmpOffset(0), _img(0), _blocks(0), _path(path) {}
 	~SubFile()
@@ -153,7 +153,7 @@ protected:
 	quint32 _gmpOffset;
 
 private:
-	IMG *_img;
+	const IMG *_img;
 	QVector<quint16> *_blocks;
 	const QString *_path;
 };
diff --git a/src/map/IMG/trefile.h b/src/map/IMG/trefile.h
index 3f17390b..0a5781db 100644
--- a/src/map/IMG/trefile.h
+++ b/src/map/IMG/trefile.h
@@ -13,9 +13,9 @@ class SubDiv;
 class TREFile : public SubFile
 {
 public:
-	TREFile(IMG *img) : SubFile(img) {}
+	TREFile(const IMG *img) : SubFile(img) {}
 	TREFile(const QString *path) : SubFile(path) {}
-	TREFile(SubFile *gmp, quint32 offset) : SubFile(gmp, offset) {}
+	TREFile(const SubFile *gmp, quint32 offset) : SubFile(gmp, offset) {}
 	~TREFile();
 
 	bool init();