diff --git a/nmpfilter/README.md b/nmpfilter/README.md
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+# nmpfilter
+
+**Purpose:**<br>
+Splits *tm-master.nmp* into smaller .nmp files filtered by region.
+
+**usage:**<br>
+`nmpfilter HwyDataDir MasterNMP OutputDir`, where
+* `HwyDataDir` is the path of the *hwy_data* directory in the HighwayData repository, or equivalent
+* `MasterNMP` is the path of *tm-master.nmp*
+* `OutputDir` is the directory in which to write the resulting .nmp files. Trailing slash required.
diff --git a/nmpfilter/nmpfilter.cpp b/nmpfilter/nmpfilter.cpp
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+++ b/nmpfilter/nmpfilter.cpp
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+// Travel Mapping Project, Eric Bryant, 2018
+#include <dirent.h>
+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+using namespace std;
+
+bool LwrCaseValid(string &RgStr)
+// Check whether a string is a valid region; convert to lower case
+{	if (RgStr == ".")		return 0;
+	if (RgStr == "..")		return 0;
+	if (RgStr == "_systems")	return 0;
+	if (RgStr == "_boundaries")	return 0;
+	for (unsigned int i = 0; i < RgStr.size(); i++)
+	  if (RgStr[i] >= 'A' && RgStr[i] <= 'Z')
+	    RgStr[i] += 32;
+	return 1;
+}
+
+void GetRegions(char *HwyDataDir, vector<string> &RgList)
+// Get a list of regions by scanning subdirectories of HwyData/
+{	DIR *dir;
+	dirent *ent;
+	if ((dir = opendir (HwyDataDir)) != NULL)
+	{	while ((ent = readdir (dir)) != NULL)
+		{	string RgStr = ent->d_name;
+			if (LwrCaseValid(RgStr)) RgList.push_back(RgStr);
+		}
+		closedir(dir);
+	}
+	else cout << "Error opening directory " << HwyDataDir << ". (Not found?)\n";
+}
+
+string GetRootRg(string RgStr)
+// Remove dashes from region codes, to match region strings as found in Roots
+{	for (int dash = RgStr.find('-'); dash > 0; dash = RgStr.find('-'))
+		RgStr.erase(dash, 1);
+	return RgStr;
+}
+
+void filter(vector<string> &RgList, char *MasterNMP, char *OutputDir)
+// Write .nmp files for each region
+{	for (unsigned int i = 0; i < RgList.size(); i++)
+	{	ifstream master(MasterNMP);
+		if (!master)
+		{	std::cout << MasterNMP << " not found\n";
+			return;
+		}
+		string pt1, pt2;
+		string outfile = OutputDir+RgList[i]+".nmp";
+		ofstream nmp(outfile.data());
+		while (getline(master, pt1) && getline(master, pt2))
+		{	string RootRg = GetRootRg(RgList[i]);
+			if (pt1.substr(0, pt1.find('.')) == RootRg || pt2.substr(0, pt2.find('.')) == RootRg)
+			nmp << pt1 << '\n' << pt2 << '\n';
+		}
+	}
+}
+
+int main(int argc, char *argv[])
+{	if (argc != 4)
+	{	cout << "usage: nmpfilter HwyDataDir MasterNMP OutputDir\n";
+		return 0;
+	}
+	vector<string> RgList;
+	GetRegions(argv[1], RgList);
+	filter(RgList, argv[2], argv[3]);
+}