diff --git a/csv.cpp b/csv.cpp
index 50f09c0..9a3acb3 100644
--- a/csv.cpp
+++ b/csv.cpp
@@ -92,3 +92,62 @@ void readcsv(const char *fn, std::vector<std::string> &header, std::map<std::str
 		exit(EXIT_FAILURE);
 	}
 }
+
+// Follow JSON rules for what looks like a number
+bool is_number(std::string const &s) {
+	const char *cp = s.c_str();
+	char c = *(cp++);
+
+	if (c == '-' || (c >= '0' && c <= '9')) {
+		if (c == '-') {
+			c = *(cp++);
+		}
+
+		if (c == '0') {
+			;
+		} else if (c >= '1' && c <= '9') {
+			c = *cp;
+
+			while (c >= '0' && c <= '9') {
+				cp++;
+				c = *cp;
+			}
+		}
+
+		if (*cp == '.') {
+			cp++;
+
+			c = *cp;
+			if (c < '0' || c > '9') {
+				return false;
+			}
+			while (c >= '0' && c <= '9') {
+				cp++;
+				c = *cp;
+			}
+		}
+
+		c = *cp;
+		if (c == 'e' || c == 'E') {
+			cp++;
+
+			c = *cp;
+			if (c == '+' || c == '-') {
+				cp++;
+			}
+
+			c = *cp;
+			if (c < '0' || c > '9') {
+				return false;
+			}
+			while (c >= '0' && c <= '9') {
+				cp++;
+				c = *cp;
+			}
+		}
+
+		return true;
+	}
+
+	return false;
+}
diff --git a/csv.hpp b/csv.hpp
index 25b3862..6c5c4bb 100644
--- a/csv.hpp
+++ b/csv.hpp
@@ -11,5 +11,6 @@ std::vector<std::string> csv_split(const char *s);
 std::string csv_dequote(std::string s);
 void readcsv(const char *fn, std::vector<std::string> &header, std::map<std::string, std::vector<std::string>> &mapping);
 std::string csv_getline(FILE *f);
+bool is_number(std::string const &s);
 
 #endif
diff --git a/jsontool.cpp b/jsontool.cpp
index ad0291c..7c4e61d 100644
--- a/jsontool.cpp
+++ b/jsontool.cpp
@@ -190,10 +190,6 @@ void out(std::string const &s, int type, json_object *properties) {
 
 std::string prev_joinkey;
 
-bool is_number(std::string const &s) {
-	return false;  // XXX
-}
-
 void join_csv(json_object *j) {
 	if (header.size() == 0) {
 		std::string s = csv_getline(csvfile);
diff --git a/tile-join.cpp b/tile-join.cpp
index c7382e7..998a971 100644
--- a/tile-join.cpp
+++ b/tile-join.cpp
@@ -207,7 +207,7 @@ void handle(std::string message, int z, unsigned x, unsigned y, std::map<std::st
 							if (joinval.size() > 0) {
 								if (joinval[0] == '"') {
 									joinval = csv_dequote(joinval);
-								} else if ((joinval[0] >= '0' && joinval[0] <= '9') || joinval[0] == '-') {
+								} else if (is_number(joinval)) {
 									attr_type = mvt_double;
 								}
 							}