Index: src/uk/me/parabola/imgfmt/app/dem/DEMFile.java
===================================================================
--- src/uk/me/parabola/imgfmt/app/dem/DEMFile.java	(revision 4070)
+++ src/uk/me/parabola/imgfmt/app/dem/DEMFile.java	(working copy)
@@ -52,20 +52,57 @@
 	 * @param outsidePolygonHeight the height value that should be used for points outside of the bounding polygon 
 	 */
 	public void calc(Area area, java.awt.geom.Area demPolygonMapUnits, String pathToHGT, List<Integer> pointDistances, short outsidePolygonHeight) {
-		HGTConverter hgtConverter = new HGTConverter(pathToHGT, area, demPolygonMapUnits);
+		// HGT area is extended by 0.01 degree in each direction
+		HGTConverter hgtConverter = new HGTConverter(pathToHGT, area, demPolygonMapUnits, 0.01D);
 		hgtConverter.setOutsidePolygonHeight(outsidePolygonHeight);
 
+		int top = area.getMaxLat() * 256;
+		int bottom = area.getMinLat() * 256;
+		int left = area.getMinLong() * 256;
+		int right = area.getMaxLong() * 256;
+
 		int zoom = 0;
 		int lastDist = pointDistances.get(pointDistances.size()-1); 
 		for (int pointDist : pointDistances) {
-			DEMSection section = new DEMSection(zoom++, area, hgtConverter, pointDist, pointDist == lastDist);
+			int distance = pointDist;
+			if (distance == -1) {
+				int res = (hgtConverter.getHighestRes() > 0) ? hgtConverter.getHighestRes() : 1200;
+				distance = (int) ((1 << 29) / (res * 45));
+			}
+			// last 4 bits of distance should be 0
+			distance = ((distance + 8)/16)*16;
+
+			int xtop = top;
+			int xleft = left;
+
+			// align DEM to distance raster, if distance not bigger than widening of HGT area
+			if (distance < (int)Math.floor((0.01D/45.0D * (1 << 29)))) {
+				if (xtop >= 0) {
+					xtop -= xtop % distance;
+					if (xtop < 0x3FFFFFFF - distance)
+						xtop += distance;
+				}
+				else {
+					xtop -= xtop % distance;
+				}
+
+				if (xleft >= 0) {
+					xleft -= xleft % distance;
+				}
+				else {
+					xleft -= xleft % distance;
+					if (xleft > Integer.MIN_VALUE + distance)
+						xleft -= distance;
+				}
+			}
+
+			// to improve: DEM secition should get area in 32bit units
+			DEMSection section = new DEMSection(zoom++, xtop, xleft, xtop - bottom, right - xleft, hgtConverter, distance, pointDist == lastDist);
 			demHeader.addSection(section);
 		}
 		return;
 	}
 
-	
-	
 	public void write() {
 		ImgFileWriter w = getWriter();
 		if (w instanceof BufferedImgFileWriter) {
Index: src/uk/me/parabola/imgfmt/app/dem/DEMSection.java
===================================================================
--- src/uk/me/parabola/imgfmt/app/dem/DEMSection.java	(revision 4070)
+++ src/uk/me/parabola/imgfmt/app/dem/DEMSection.java	(working copy)
@@ -48,16 +48,12 @@
 	private int maxHeight = Integer.MIN_VALUE;
 	private List<DEMTile> tiles = new ArrayList<>();
 	
-	public DEMSection(int zoomLevel, Area bbox, HGTConverter hgtConverter, int pointDist, boolean lastLevel) {
+	public DEMSection(int zoomLevel, int areaTop, int areaLeft, int areaHeight, int areaWidth, HGTConverter hgtConverter, int pointDist, boolean lastLevel) {
 		this.zoomLevel = zoomLevel;
 		this.lastLevel = lastLevel;
 		
-		if (pointDist == -1) {
-			int res = (hgtConverter.getHighestRes() > 0) ? hgtConverter.getHighestRes() : 1200;
-			pointDist = (int) ((1 << 29) / (res * 45));
-		}
-		this.top = bbox.getMaxLat() * 256;
-		this.left = bbox.getMinLong() * 256;
+		this.top = areaTop;
+		this.left = areaLeft;
 
 		// calculate raster that starts at top left corner
 		// last row and right column have non-standard height / row values 
@@ -64,8 +60,8 @@
 		pointsDistanceLat = pointDist; 
 		pointsDistanceLon = pointDist;
 		
-		int []latInfo = getTileInfo(bbox.getHeight() * 256, pointsDistanceLat);
-		int []lonInfo = getTileInfo(bbox.getWidth() * 256, pointsDistanceLon);
+		int []latInfo = getTileInfo(areaHeight, pointsDistanceLat);
+		int []lonInfo = getTileInfo(areaWidth, pointsDistanceLon);
 		// store the values written to the header
 		tilesLat = latInfo[0];
 		tilesLon = lonInfo[0];
Index: src/uk/me/parabola/mkgmap/reader/hgt/HGTConverter.java
===================================================================
--- src/uk/me/parabola/mkgmap/reader/hgt/HGTConverter.java	(revision 4070)
+++ src/uk/me/parabola/mkgmap/reader/hgt/HGTConverter.java	(working copy)
@@ -46,14 +46,18 @@
 	 * @param demPolygonMapUnits optional bounding polygon which describes the area for 
 	 * which elevation should be read from hgt files.
 	 */
-	public HGTConverter(String path, Area bbox, java.awt.geom.Area demPolygonMapUnits) {
-		int minLat = (int) Math.floor(Utils.toDegrees(bbox.getMinLat()));
-		int minLon = (int) Math.floor(Utils.toDegrees(bbox.getMinLong()));
-		// the bbox is slightly enlarged so that we dont fail when rounding has no effect
-		// e.g. if getMaxLat() returns 0
-		int maxLat = (int) Math.ceil(Utils.toDegrees(bbox.getMaxLat() + 1));
-		int maxLon = (int) Math.ceil(Utils.toDegrees(bbox.getMaxLong() + 1));
+	public HGTConverter(String path, Area bbox, java.awt.geom.Area demPolygonMapUnits, double extra) {
+		// make bigger box for interpolation or aligning of areas
+		int minLat = (int) Math.floor(Utils.toDegrees(bbox.getMinLat()) - extra);
+		int minLon = (int) Math.floor(Utils.toDegrees(bbox.getMinLong()) - extra);
+		int maxLat = (int) Math.ceil(Utils.toDegrees(bbox.getMaxLat()) + extra);
+		int maxLon = (int) Math.ceil(Utils.toDegrees(bbox.getMaxLong()) + extra);
 
+		if (minLat < -90) minLat = -90;
+		if (maxLat > 90) maxLat = 90;
+		if (minLon < -180) minLon = -180;
+		if (maxLon > 180) maxLon = 180;
+
 		minLat32 = Utils.toMapUnit(minLat) * 256; 
 		minLon32 = Utils.toMapUnit(minLon) * 256; 
 		// create matrix of readers