First let's look at the units of measure in these two documents.
The first transcription (1834) says Francis Green (FG) occupies four fields of 12 acres.
The second document (1828) says Hugh Biddick (HB) occupies two fields of 7A. 3R. 35P., while FB occupies house, mowhay, and two fields of 5A. 0R. 23P.
A. R. P. are a historically used system of area measurement: acres, roods, and perches, based on the rod.
- A rod is a unit of length equal to 16.5 survey feet.
- A survey foot is 12 inches, and there are 39.37 inches in 1 meter.
- A perch is a unit of area equal to one square rod.
- A rood is 40 perches.
- An acre is 4 roods, equal to 160 perches, or 160 square rods, or 43,560 square feet, or 1/640th of a square mile.
Summing the areas in the second document, we get 7A. 3R. 35P. + 5A. 0R. 23P. = 12A. 3R. 58P. = 12A. 4R. 18P. = 13A. 0R. 18P. = 13.1125 acres. So the items in the second document sum to house, mowhay, and four fields of 13A. 0R. 18P. Is it possible that FB came to occupy the two fields that HB was previously occupying? But there is an extra acre to be accounted for.
Now let's look at the location. We already know we are looking at Truro, Cornwall. Both documents mention St Clement: Poor House of St Clement, lane to St Clement Church Town, Parish of St. Clement.
The Kresen Kernow website's Mapping page says they're still working on digitising their map collections. Their map Collections Guide says:
The first extensive mapping of Cornwall took place around 1840 when tithe maps were drawn up of each parish so that landowners knew how much tithe (a tax) they could charge. The maps are accompanied by an apportionment book which gives information including who rented each plot. The maps can be very useful for property history and have all been digitised.
Ordnance Survey maps are vital for understanding how Cornwall has changed over time. Kresen Kernow holds a wide selection of these maps dating from 1813 and later editions, including 1880, 1907, 1930s, 1960s and 1970s, some of which are at a large scale. The National Library of Scotland has made the Ordnance Survey map collection available to search online.
This lets us know that Kresen Kernow may have maps for the right time period, and that the maps may also be at the NLS.
At the top of most pages of the Kresen Kernow site, there is a box labeled "Explore our collections" with a search button. Perform a search for "St Clement" with the quotation marks; otherwise you'll get every St in the collection. The results page shows a couple thousand results for "St Clement," but these are all formats of materials from all time periods. To limit the results, scroll to "Formats" and select "Map" and "Update." If you like, you can limit by year to a range of years, e.g. 1800-1900.
The results show that Kresen Kernow possesses about five tithe maps of St Clement from the 1840-1844 time period, which might be sufficiently close to show names such as "Parkengrowse" or "Parkengrouse." Clicking on each of these, however, we find more information about how each map was constructed. We also find that for these maps, the images are not online at this site. (We can also see this by limiting our search to results with images.)
Make a careful note of the "Reference number" for each map that looks useful. In the website menu, under Collections, select "Copies and research." Here we find that (emphasis mine)
We are able to provide copies (digital or printed) of many of our documents, and carry out research for you. We can also provide copies of tithe maps on CD.
There is a link to a Tithe map CD order form (.docx) and a list of their charges, which run £10 per image. However, according to the order form, the tithe map CD is a wealth of information:
Each tithe map package consists of a digitised copy of the tithe map and tithe apportionment (dated c1840) for a specific parish. . . . The information in the original apportionments is arranged alphabetically by landowner. To make it easier to find entries, these documents have been transcribed into Excel spreadsheets and the information sorted both by plot number and landholding. The data can also be searched using the spreadsheet’s search functions.
Turning now to the National Library of Scotland, we find that they have ordnance survey maps of England and Wales, 1841-1952, at a scale of 25 inches to the mile and 6 inches to the mile. Inside one of these map finders, you can select county = Cornwall and parish = St Clement.
For St Clement, the earliest maps here appear to be from around 1878. They show property lines, and the 25-inch maps show parcel numbers, acreage of parcels, and a few names. You can find the Truro Union Workhouse, i.e. poor house, and you can find St Clement Church. Some parcels, such as a 13.076-acre parcel southwest of the church, are close in size to the one you're looking for, but of course parcels are combined and split over 40 years, and you would need the apportionment to be sure who occupied the land.
The NLS site would be useful if you had a parcel number.
It may be worth it to examine the descriptions of the tithe maps at Kresen Kernow and order a CD of the one that seems most promising. While you're on that site, you can also search for other documents they might have related to land taxation/title/leases that you could order at the same time.