[Ger-Poland-Volhynia] Gottfried Hohertz from Zrawin

[j burns]

I've started researching a brother-in-law's ancestors and don't know where exactly they came from. 
His grandfather, Gottfried Hohertz, was born 1861 March 6 in Russia. He immigrated to Canada via the US in 1910. On a US border crossing record in 1911 he says he was born in what looks like Zrawin, Russia (Zraivin?). About 1889 he married Pauline Herman (daughter of George Karl Herman and Karoline Obst) who was born 1866 Aug 15 at Shitomir, Volynia, Russia (this information comes from a second cousin of my brother-in-law-she hasn't told me where she got it). They had 7 children in Russia;
Lydia 1890 Oct 19, Wilhelm 1896 Mar 22, Emil 1898 Sep 18, Wanda 1900, Marno 1904 Dec., Gustav 1906 Jan 11, and Fredrick 1901 Aug 26. The above mentioned second cousin says the children were all born in Dubrokuched, Volynia, Russia-again she hasn't told me where she got this information. I have the death certificate for the above mentioned son Gustav and it says he was born in Dobrokud, Russia. Some later descendents say some of the above children were born in Poland. One thing they all agree on is that the Hohertz were of German origin.
I've have tried searching some maps to find Zrawin and Dobrokud-so far I haven't had any luck. Has anyone heard of these places or can anyone point me towards a map where I could find them? 
I have taken a brief look at the website for The Society of German Genealogy in Eastern Europe but it costs $40 to join and I am not certain the Hohertz were from there. 
Does any have suggestions as to where I should be looking or other sites or?
John