Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Pinnacle height | Year | Structure type | Main use | Country | Town |
| --- | --- | --- | --- | --- | --- |
| m (ft) | 1969.0 | Guyed mast | VHF-UHF transmission | U.S. | Hemingford, Nebraska |
| m (ft) | 1992.0 | Guyed mast | VHF-UHF transmission | U.S. | Liverpool, Texas |
| m (ft) | 2001.0 | Guyed mast | VHF-UHF transmission | U.S. | Dillon, South Carolina |
| m (ft) | 1990.0 | Guyed mast | VHF-UHF transmission | U.S. | Bertram, Texas |
| m (ft) | 2002.0 | Guyed mast | VHF-UHF transmission | U.S. | Collinsville, Texas |
| m (ft) (orig. 609.6 m) | 1971.0 | Guyed mast | VHF-UHF transmission | U.S. | Fordland, Missouri |
| m (ft) | 1981.0 | Guyed mast | VHF-UHF transmission | U.S. | Winnabow, North Carolina |
| m (ft) | 2001.0 | Guyed mast | VHF-UHF transmission | U.S. | Robertsdale, Alabama |
| m (ft) | 1985.0 | Guyed mast | VHF-UHF transmission | U.S. | Dallas, North Carolina |
| m (ft) | 1977.0 | Guyed mast | VHF-UHF transmission | U.S. | Grady, Alabama |
| m (ft) | 1986.0 | Guyed mast | VHF-UHF transmission | U.S. | Columbia, Louisiana |
| m (ft) | 1988.0 | Guyed mast | VHF-UHF transmission | U.S. | Splendora, Texas |
| m (ft) | 1974.0 | Guyed mast | VHF-UHF transmission | U.S. | Walker City, Iowa |
| m (ft) | 1981.0 | Guyed mast | VHF-UHF transmission | U.S. | Eddy, Texas |
| m (ft) | 1998.0 | Guyed mast | VHF-UHF transmission | U.S. | Greenwood, Texas |
| m (ft) | 1999.0 | Guyed mast | VHF-UHF transmission | U.S. | Rio Grande City, Texas |
| m (ft) | 2004.0 | Guyed mast | VHF-UHF transmission | U.S. | Randleman, North Carolina |
| m (ft) | 1988.0 | Guyed mast | VHF-UHF transmission | U.S. | Coweta, Oklahoma |
| m (ft) | 2004.0 | Guyed mast | VHF-UHF transmission | U.S. | Robertsdale, Alabama |
| m (ft) | 1998.0 | Offshore platform | Oil drilling | U.S. | Garden Banks , Gulf of Mexico |
| m (ft) | 1978.0 | Guyed mast | VHF-UHF transmission | U.S. | Bethlehem, Florida |
| m (ft) | 1985.0 | Guyed mast | VHF-UHF transmission | U.S. | Redfield, Arkansas |
| m (ft) | 2002.0 | Guyed mast | VHF-UHF transmission | U.S. | Greensboro, North Carolina |
| m (ft) | 2000.0 | Guyed mast | VHF-UHF transmission | U.S. | Shepherd, Texas |
| m (ft) | 1986.0 | Guyed mast | VHF-UHF transmission | U.S. | Spanish Fort, Alabama |
| m (ft) | 2000.0 | Guyed mast | VHF-UHF transmission | U.S. | Saint Cloud, Florida |
| m (ft) | 1980.0 | Guyed mast | VHF-UHF transmission | U.S. | Gray Court, South Carolina |
| m (ft) | 1965.0 | Guyed mast | VHF-UHF transmission | U.S. | Genoa, Nebraska |
| m (ft) | 1993.0 | Guyed mast | VHF-UHF transmission | U.S. | Princeton, Florida |
| m (ft) | 1985.0 | Guyed mast | VHF-UHF transmission | U.S. | Gretna, Nebraska |
| m (ft) | 1965.0 | Guyed mast | VHF-UHF transmission | U.S. | Roswell, New Mexico |
| m (ft) | 1984.0 | Guyed mast | VHF-UHF transmission | U.S. | Oneta, Oklahoma |
| m (ft) | 2003.0 | Guyed mast | VHF-UHF transmission | U.S. | Shreveport, Louisiana |
| m (ft) | 1976.0 | Concrete tower | Observation, VHF-UHF transmission | Canada | Toronto, Ontario |
Question:
What is the main use for the structure listed in walker city, iowa?
Answer:
VHF-UHF transmission