Assuming that you already have Geocodes added for the Person Account record, you can access the latitude and longitude fields in SOQL query as for any account record. Your query should look like (say if you are fetching the values from Billing Address) as:
Account a = [select Id, Name, BillingLatitude, BillingLongitude from Account where id = 'xxxxx' limit 1];
For api names on the fields and on which addresses those are supported, you can find more on the help link.
Geocodes for Person Accounts cannot be added using data integration rules, for that purpose you will need to use API or third-party services. Below is an excerpt from the considerations for Person Account.
Geocodes for addresses can’t be added to person accounts using data integration rules. Use the API or third-party services to add geocode data to person accounts.
Additional information can be found on this link on Geolocation custom fields.
To be able to add geocodes on the record, you can set those fields as regular fields. An example below.
Account a = [select id, BillingLatitude, BillingLongitude from account where id = 'xxx' limit 1];
a.BillingLatitude = <value>;
a.BillingLongitude = <value>;