Final answer:
To effectively represent two quantitative values on a symbol map, the first value should be represented with size and the second with color, along with standard map elements such as scale bar and symbol explanations.
Step-by-step explanation:
When creating symbol maps that show two quantitative values per location, it is important to use a clear and effective representation method. An appropriate way to represent these values would be to encode the first value with size and the second value with color. This method allows you to differentiate between the two separate data sets visually, ensuring that they can be interpreted accurately. Encoding positive and negative values using size and color respectively may not effectively represent two different quantitative values, as it conflates their distinctions into a single framework of positive/negative. Using varying shades of the same color for both values could be confusing and not distinct enough for clear visualization. Lastly, converting the values into latitude and longitude is irrelevant for the visualization of quantitative data on a symbol map and should not be considered a viable method.
The principles of map creation suggest that a good map should include a scale bar that shows the ratio of distance on the map to distance on the ground, represented in both kilometers and miles. It should also have a north arrow, perhaps latitude and longitude, and an explanation of any symbols including colors, lines, or icons used on the map. These are essential elements for communicating spatial data about the Earth's surface, which map designers should keep in mind.